test_defer_excchain_dump_ipython was setting fresh env anew, thus not
propagating $LD_PRELOAD that trun sets when libgolang was built with
thread or address sanitizer. This way the subprocess spawned for IPython
was failing with e.g.

        ==152924==ASan runtime does not come first in initial library list; you should either link runtime to your application or manually preload it with LD_PRELOAD.

-> Fix it by only adjusting environment for spawned IPython instead of
setting it completely anew.

Fixes: 09629367 (golang: tests: Add tests for IPython and Pytest
integration patches)

For example if test_defer_excchain_dump_ipython fails the output was:

    >       assert retcode == 0
    E       assert 1 == 0

and it was unclear what was going on. Now the output is e.g.

    >       assert retcode == 0, (stdout, stderr)
    E       AssertionError: ('', '==152924==ASan runtime does not come first in initial library list; you should either link runtime to your application or manually preload it with LD_PRELOAD.
    E         ')
    E       assert 1 == 0

Amends: 09629367 (golang: tests: Add tests for IPython and Pytest
integration patches) and bb9a94c3 (golang: Teach defer to chain
exceptions (PEP 3134) even on Python2).

Until now gpython was always activating gevent on startup + adding
goodness such as "always UTF-8 default encoding"; go, chan, b/u and
friends available from builtin namespace, etc... While those goodness
are sometimes useful on their own, it is not always appropriate to force
a project to switch from threads to gevent.

For this reason add a flag to select which runtime - either gevent
or threads - gpython should use.

	gpython -Xgpython.runtime=gevent	selects gevent,

while

	gpython -Xgpython.runtime=threads	selects threads.

Gevent remains the default.

It is also possible to specify desired runtime via $GPYTHON_RUNTIME
environment variable.

/reviewed-on nexedi/pygolang!5

Restructure code a bit to prepare it for the next patch.
Plain code movement.

/reviewed-on nexedi/pygolang!5

On CPython:

	$ python -V
	Python 2.7.18

	$ gpython -V
	GPython 0.0.6.post2 [gevent 20.6.2] / CPython 2.7.18

On PyPy:

	$ python -V
        Python 3.6.9 (2ad108f17bdb, Apr 07 2020, 02:59:05)
        [PyPy 7.3.1 with GCC 7.3.1 20180303 (Red Hat 7.3.1-5)]

	$ gpython -V
	GPython 0.0.6.post2 [gevent 20.5.0] / PyPy 7.3.1 / Python 3.6.9

/reviewed-on !5

And same for -m<module> - becuase Python supports it this way.

Before the patch:

	$ python '-cprint "hello world"'
	hello world
	$ gpython '-cprint "hello world"'
	unknown option: '-cprint "hello world"'

After the patch:

	$ python '-cprint "hello world"'
	hello world
	$ gpython '-cprint "hello world"'
	hello world

/reviewed-on nexedi/pygolang!5

	kirr@deco:~$ python3.8 -v
	import _frozen_importlib # frozen
	import _imp # builtin
	import '_thread' # <class '_frozen_importlib.BuiltinImporter'>
	import '_warnings' # <class '_frozen_importlib.BuiltinImporter'>
	import '_weakref' # <class '_frozen_importlib.BuiltinImporter'>
	import '_frozen_importlib_external' # <class '_frozen_importlib.FrozenImporter'>
	import '_io' # <class '_frozen_importlib.BuiltinImporter'>
	import 'marshal' # <class '_frozen_importlib.BuiltinImporter'>
	import 'posix' # <class '_frozen_importlib.BuiltinImporter'>
	import _thread # previously loaded ('_thread')
	import '_thread' # <class '_frozen_importlib.BuiltinImporter'>
	import _weakref # previously loaded ('_weakref')
	import '_weakref' # <class '_frozen_importlib.BuiltinImporter'>
	# installing zipimport hook
	import 'time' # <class '_frozen_importlib.BuiltinImporter'>		<-- NOTE
	import 'zipimport' # <class '_frozen_importlib.FrozenImporter'>
	# installed zipimport hook

See https://github.com/python/cpython/commit/79d1c2e6c9d1 for details
(`import time` in zipimport.py)

https://github.com/gevent/gevent/issues/1461 was fixed in
https://github.com/gevent/gevent/commit/cc7e28cf

Gevent is major component gpython builds on - it deserves to be included
into version and is handy to know if one hits a bug with gpython - to
see in which particular runtime environment the bug was hit.

Before:

    $ gpython
    Python 3.7.3 (default, Dec 20 2019, 18:57:59)
    [GCC 8.3.0] [GPython 0.0.6.post2] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    (InteractiveConsole)
    >>>

After:

    $ gpython
    Python 3.7.3 (default, Dec 20 2019, 18:57:59)
    [GCC 8.3.0] [GPython 0.0.6.post2] [gevent 20.5.1] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    (InteractiveConsole)
    >>>

/cc @gabriel, @jerome
/proposed-for-review-on: nexedi/pygolang!4

Add tests for patches to third-party software that were added in
bb9a94c3.

/cc @gabriel, @jerome
/reviewed-on: !2

This makes recovered exception object a good target to be explicitly
chained into (PEP 3134) via .__cause__ because without .__traceback__ the
information about where/how recovered exception originated would be lost.

In turn, having recover to always return full exception info, should
allow to use recover in "add error context"-style utilities.

/reviewed-on: nexedi/pygolang!3

Starting from setuptools-dso 1.7 pyx.build no longer misses to
(re)compile dsos when `build_ext -i` is run:

https://github.com/mdavidsaver/setuptools_dso/pull/10
https://github.com/mdavidsaver/setuptools_dso/commit/84e06348

Previously `pip install -e` would just fail when run afresh because DSOs
were not built, and so link ext->dso was failing.

bb9a94c3 (golang: Teach defer to chain exceptions (PEP 3134) even on
Python2) added integration patches for IPython and Pytest to properly
dump tracebacks for chained exceptions even on Python2. However the
functionality of patches was tested only manually.

-> Add corresponding tests to verify how IPython and Pytest behaves
when dumping tracebacks.

assertDoc normalizes paths in compared texts with the idea for etalon
output to contain PYGOLANG instead of whatever actual path there will be
when testing the package. This already works.

However IPython, when dumping tracebacks, tries to shorten paths and
abbreviate $HOME with ~ in them. This breaks normalization which misses
to convert prefix of those paths into PYGOLANG.

-> Fix it by teaching assertDoc to also handle paths that start with ~
and correctly normalize them.

This will be needed in the next patch where we will add tests for how
ipython and pytest dump tracebacks for chained exceptions.

We are going to use it from several places.

Paths to the directories are already used in several functions, and are
going to be used more. Move them to common place to avoid duplication.

Starting from setuptools-dso 1.6 pyx.build no longer fails on macOS due to:

  https://github.com/mdavidsaver/setuptools_dso/commit/dd2cf303
  https://github.com/mdavidsaver/setuptools_dso/commit/6883d6dc
  https://github.com/mdavidsaver/setuptools_dso/commit/78ae8852
  https://github.com/mdavidsaver/setuptools_dso/pull/9
  https://github.com/mdavidsaver/setuptools_dso/issues/8

It was previously failing with setuptools-dso==1.5:

  (py3.venv) kirr@Kirills-iMac pygolang % python -m pytest
  ==================================== test session starts =====================================
  platform darwin -- Python 3.7.6, pytest-5.3.5, py-1.8.1, pluggy-0.13.1
  rootdir: /Users/kirr/pygolang
  collected 100 items

  golang/_gopath_test.py ..                                                              [  2%]
  golang/context_test.py ..                                                              [  4%]
  golang/cxx_test.py ..                                                                  [  6%]
  golang/errors_test.py ........                                                         [ 14%]
  golang/fmt_test.py ...                                                                 [ 17%]
  golang/golang_test.py .............................................                    [ 62%]
  golang/io_test.py .                                                                    [ 63%]
  golang/strconv_test.py ..                                                              [ 65%]
  golang/strings_test.py .....                                                           [ 70%]
  golang/sync_test.py .............                                                      [ 83%]
  golang/time_test.py ........                                                           [ 91%]
  golang/pyx/build_test.py FF.                                                           [ 94%]
  golang/pyx/runtime_test.py .                                                           [ 95%]
  gpython/gpython_test.py ssss.                                                          [100%]

  ========================================== FAILURES ==========================================
  _______________________________________ test_pyx_build _______________________________________

      def test_pyx_build():
          pyxuser = testprog + "/golang_pyx_user"
          pyrun(["setup.py", "build_ext", "-i"], cwd=pyxuser)

          # run built test.
          _ = pyout(["-c",
              # XXX `import golang` is a hack: it dynamically loads _golang.so -> libgolang.so,
              # and this way dynamic linker already has libgolang.so DSO found and in
              # memory for all further imports. If we don't, current state of setuptools_dso
              # is that `import pyxuser.test` will fail finding libgolang.so.
              "import golang;" +
  >           "from pyxuser import test; test.main()"], cwd=pyxuser)

  golang/pyx/build_test.py:40:
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
  golang/golang_test.py:1709: in pyout
      return pyrun(argv, stdin=stdin, stdout=stdout, stderr=stderr, **kw)
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

  argv = ['-c', 'import golang;from pyxuser import test; test.main()'], stdin = None, stdout = b'', stderr = None
  kw = {'cwd': '/Users/kirr/pygolang/golang/pyx/testprog/golang_pyx_user'}, retcode = 1

      def pyrun(argv, stdin=None, stdout=None, stderr=None, **kw):
          retcode, stdout, stderr = _pyrun(argv, stdin=stdin, stdout=stdout, stderr=stderr, **kw)
          if retcode:
  >           raise RuntimeError(' '.join(argv) + '\n' + (stderr and str(stderr) or '(failed)'))
  E           RuntimeError: -c import golang;from pyxuser import test; test.main()
  E           (failed)

  golang/golang_test.py:1703: RuntimeError
  ------------------------------------ Captured stdout call ------------------------------------
  running build_ext
  cythoning pyxuser/test.pyx to pyxuser/test.cpp
  building 'pyxuser.test' extension
  creating build
  creating build/temp.macosx-10.15-x86_64-3.7
  creating build/temp.macosx-10.15-x86_64-3.7/pyxuser
  clang -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/usr/include -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/Tk.framework/Versions/8.5/Headers -I/Users/kirr/pygolang -I/Users/kirr/py3.venv/bin/../include/site/python3.7 -I/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/include/python3.7m -c pyxuser/test.cpp -o build/temp.macosx-10.15-x86_64-3.7/pyxuser/test.o -std=c++11 -fno-strict-aliasing
  creating build/lib.macosx-10.15-x86_64-3.7
  creating build/lib.macosx-10.15-x86_64-3.7/pyxuser
  clang++ -bundle -undefined dynamic_lookup build/temp.macosx-10.15-x86_64-3.7/pyxuser/test.o -L/Users/kirr/pygolang/golang/runtime -llibgolang -o build/lib.macosx-10.15-x86_64-3.7/pyxuser/test.cpython-37m-darwin.so
  otool -L build/lib.macosx-10.15-x86_64-3.7/pyxuser/test.cpython-37m-darwin.so
  build/lib.macosx-10.15-x86_64-3.7/pyxuser/test.cpython-37m-darwin.so:
  	@loader_path/liblibgolang.0.1.dylib (compatibility version 0.0.0, current version 0.0.0)
  	/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 800.7.0)
  	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1281.0.0)
  install_name_tool -change @loader_path/liblibgolang.0.1.dylib @loader_path/../golang/runtime/liblibgolang.0.1.dylib build/lib.macosx-10.15-x86_64-3.7/pyxuser/test.cpython-37m-darwin.so
  otool -L build/lib.macosx-10.15-x86_64-3.7/pyxuser/test.cpython-37m-darwin.so
  build/lib.macosx-10.15-x86_64-3.7/pyxuser/test.cpython-37m-darwin.so:
  	@loader_path/../golang/runtime/liblibgolang.0.1.dylib (compatibility version 0.0.0, current version 0.0.0)
  	/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 800.7.0)
  	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1281.0.0)
  copying build/lib.macosx-10.15-x86_64-3.7/pyxuser/test.cpython-37m-darwin.so -> pyxuser
  ------------------------------------ Captured stderr call ------------------------------------
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
  ImportError: dlopen(/Users/kirr/pygolang/golang/pyx/testprog/golang_pyx_user/pyxuser/test.cpython-37m-darwin.so, 2): Library not loaded: @loader_path/../golang/runtime/liblibgolang.0.1.dylib
    Referenced from: /Users/kirr/pygolang/golang/pyx/testprog/golang_pyx_user/pyxuser/test.cpython-37m-darwin.so
    Reason: image not found
  _______________________________________ test_dso_build _______________________________________

      def test_dso_build():
          dsouser = testprog + "/golang_dso_user"
          pyrun(["setup.py", "build_dso", "-i"], cwd=dsouser)
          pyrun(["setup.py", "build_ext", "-i"], cwd=dsouser)

          # run built test.
          _ = pyout(["-c",
              # XXX `import golang` is a hack - see test_pyx_build for details.
              "import golang;" +
  >           "from dsouser import test; test.main()"], cwd=dsouser)

  golang/pyx/build_test.py:54:
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
  golang/golang_test.py:1709: in pyout
      return pyrun(argv, stdin=stdin, stdout=stdout, stderr=stderr, **kw)
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

  argv = ['-c', 'import golang;from dsouser import test; test.main()'], stdin = None, stdout = b'', stderr = None
  kw = {'cwd': '/Users/kirr/pygolang/golang/pyx/testprog/golang_dso_user'}, retcode = 1

      def pyrun(argv, stdin=None, stdout=None, stderr=None, **kw):
          retcode, stdout, stderr = _pyrun(argv, stdin=stdin, stdout=stdout, stderr=stderr, **kw)
          if retcode:
  >           raise RuntimeError(' '.join(argv) + '\n' + (stderr and str(stderr) or '(failed)'))
  E           RuntimeError: -c import golang;from dsouser import test; test.main()
  E           (failed)

  golang/golang_test.py:1703: RuntimeError
  ------------------------------------ Captured stdout call ------------------------------------
  running build_dso
  Building DSOs
  building 'dsouser.dso' DSO as build/lib.macosx-10.15-x86_64-3.7/dsouser/libdso.dylib
  creating build
  creating build/temp.macosx-10.15-x86_64-3.7
  creating build/temp.macosx-10.15-x86_64-3.7/dsouser
  clang -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/usr/include -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/Tk.framework/Versions/8.5/Headers -I/Users/kirr/pygolang -c dsouser/dso.cpp -o build/temp.macosx-10.15-x86_64-3.7/dsouser/dso.o -std=c++11 -fno-strict-aliasing
  creating build/lib.macosx-10.15-x86_64-3.7
  creating build/lib.macosx-10.15-x86_64-3.7/dsouser
  clang++ -dynamiclib -undefined dynamic_lookup build/temp.macosx-10.15-x86_64-3.7/dsouser/dso.o -L/Users/kirr/pygolang/golang/runtime -llibgolang -o build/lib.macosx-10.15-x86_64-3.7/dsouser/libdso.dylib -install_name @loader_path/libdso.dylib
  otool -L build/lib.macosx-10.15-x86_64-3.7/dsouser/libdso.dylib
  build/lib.macosx-10.15-x86_64-3.7/dsouser/libdso.dylib:
  	@loader_path/libdso.dylib (compatibility version 0.0.0, current version 0.0.0)
  	@loader_path/liblibgolang.0.1.dylib (compatibility version 0.0.0, current version 0.0.0)
  	/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 800.7.0)
  	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1281.0.0)
  install_name_tool -change @loader_path/liblibgolang.0.1.dylib @loader_path/../golang/runtime/liblibgolang.0.1.dylib build/lib.macosx-10.15-x86_64-3.7/dsouser/libdso.dylib
  otool -L build/lib.macosx-10.15-x86_64-3.7/dsouser/libdso.dylib
  build/lib.macosx-10.15-x86_64-3.7/dsouser/libdso.dylib:
  	@loader_path/libdso.dylib (compatibility version 0.0.0, current version 0.0.0)
  	@loader_path/../golang/runtime/liblibgolang.0.1.dylib (compatibility version 0.0.0, current version 0.0.0)
  	/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 800.7.0)
  	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1281.0.0)
  copying build/lib.macosx-10.15-x86_64-3.7/dsouser/libdso.dylib -> dsouser
  running build_ext
  cythoning dsouser/test.pyx to dsouser/test.cpp
  building 'dsouser.test' extension
  clang -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/usr/include -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/Tk.framework/Versions/8.5/Headers -I/Users/kirr/pygolang -I/Users/kirr/py3.venv/bin/../include/site/python3.7 -I/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/include/python3.7m -c dsouser/test.cpp -o build/temp.macosx-10.15-x86_64-3.7/dsouser/test.o -std=c++11 -fno-strict-aliasing
  clang++ -bundle -undefined dynamic_lookup build/temp.macosx-10.15-x86_64-3.7/dsouser/test.o -L/Users/kirr/pygolang/golang/runtime -Lbuild/lib.macosx-10.15-x86_64-3.7/dsouser -llibgolang -ldso -o build/lib.macosx-10.15-x86_64-3.7/dsouser/test.cpython-37m-darwin.so
  otool -L build/lib.macosx-10.15-x86_64-3.7/dsouser/test.cpython-37m-darwin.so
  build/lib.macosx-10.15-x86_64-3.7/dsouser/test.cpython-37m-darwin.so:
  	@loader_path/liblibgolang.0.1.dylib (compatibility version 0.0.0, current version 0.0.0)
  	@loader_path/libdso.dylib (compatibility version 0.0.0, current version 0.0.0)
  	/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 800.7.0)
  	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1281.0.0)
  install_name_tool -change @loader_path/liblibgolang.0.1.dylib @loader_path/../golang/runtime/liblibgolang.0.1.dylib build/lib.macosx-10.15-x86_64-3.7/dsouser/test.cpython-37m-darwin.so
  install_name_tool -change @loader_path/libdso.dylib @loader_path/./libdso.dylib build/lib.macosx-10.15-x86_64-3.7/dsouser/test.cpython-37m-darwin.so
  otool -L build/lib.macosx-10.15-x86_64-3.7/dsouser/test.cpython-37m-darwin.so
  build/lib.macosx-10.15-x86_64-3.7/dsouser/test.cpython-37m-darwin.so:
  	@loader_path/../golang/runtime/liblibgolang.0.1.dylib (compatibility version 0.0.0, current version 0.0.0)
  	@loader_path/./libdso.dylib (compatibility version 0.0.0, current version 0.0.0)
  	/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 800.7.0)
  	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1281.0.0)
  copying build/lib.macosx-10.15-x86_64-3.7/dsouser/test.cpython-37m-darwin.so -> dsouser
  ------------------------------------ Captured stderr call ------------------------------------
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
  ImportError: dlopen(/Users/kirr/pygolang/golang/pyx/testprog/golang_dso_user/dsouser/test.cpython-37m-darwin.so, 2): Library not loaded: @loader_path/../golang/runtime/liblibgolang.0.1.dylib
    Referenced from: /Users/kirr/pygolang/golang/pyx/testprog/golang_dso_user/dsouser/test.cpython-37m-darwin.so
    Reason: image not found
  ========================== 2 failed, 94 passed, 4 skipped in 20.26s ==========================

qq is used to quote strings or byte-strings. The following example
illustrates the problem we are currently hitting in zodbtools with
Python3:

    >>> "hello %s" % qq("мир")
    'hello "мир"'

    >>> b"hello %s" % qq("мир")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: %b requires a bytes-like object, or an object that implements __bytes__, not 'str'

    >>> "hello %s" % qq(b("мир"))
    'hello "мир"'

    >>> b"hello %s" % qq(b("мир"))
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: %b requires a bytes-like object, or an object that implements __bytes__, not 'str'

i.e. one way or another if type of format string and what qq returns do not
match it creates a TypeError.

We want qq(obj) to be useable with both string and bytestring format.

For that let's teach qq to return special str- and bytes- derived types that
know how to automatically convert to str->bytes and bytes->str via b/u
correspondingly. This way formatting works whatever types combination it was
for format and for qq, and the whole result has the same type as format.

For now we teach only qq to use new types and don't generally expose
_str and _unicode to be returned by b and u yet. However we might do so
in the future after incrementally gaining a bit more experience.

/proposed-for-review-on: nexedi/pygolang!1

i.e. unicode on py3 and bytes on py2. This makes it more likely to work
for

	"format string %s ..." % qq(object)

with format string being str and object of arbitrary type.

However more on this topic in the next patch.

i.e. b(b(·)) and u(u(·)) are always identity.

This property already holds and is easy to verify just by code review.
However it might become less obvious once we start to tweak b and u.

-> Add explicit test.

The new home is https://lab.nexedi.com/nexedi/pygolang

[1] https://stack.nexedi.com

A build fix wrt gevent-1.5 + benchmarks for nogil go and channels.

Starting from gevent >= 1.5 '*.pxd' files for gevent API are no longer
provided, at least in released gevent wheels. This broke pygolang:

    Error compiling Cython file:
    ------------------------------------------------------------
    ...

    # Gevent runtime uses gevent's greenlets and semaphores.
    # When sema.acquire() blocks, gevent switches us from current to another greenlet.

    IF not PYPY:
        from gevent._greenlet cimport Greenlet
       ^
    ------------------------------------------------------------

    golang/runtime/_runtime_gevent.pyx:28:4: 'gevent/_greenlet.pxd' not found

Since gevent upstream refuses to restore Cython level access[1], let's fix the
build by using gevent bits via Python-level.

Even when used via py import gevent-1.5 brings speed improvement compared to
gevent-1.4 (used via cimport):

	(on i7@2.6GHz, gevent runtime)

                      gevent-1.4   gevent-1.5
                      (cimport)    (py import)

    name              old time/op  new time/op  delta
    pyx_select_nogil  9.47µs ± 0%  8.74µs ± 0%   -7.70%  (p=0.000 n=10+9)
    pyx_go_nogil      14.3µs ± 1%  12.0µs ± 1%  -16.52%  (p=0.000 n=10+10)
    pyx_chan_nogil    7.10µs ± 1%  6.32µs ± 1%  -10.89%  (p=0.000 n=10+10)
    go                16.0µs ± 2%  13.4µs ± 1%  -16.37%  (p=0.000 n=10+10)
    chan              7.50µs ± 0%  6.79µs ± 0%   -9.53%  (p=0.000 n=10+10)
    select            10.8µs ± 1%  10.0µs ± 1%   -6.78%  (p=0.000 n=10+10)

Using gevent-1.5 could have been even faster via cimport (it is still
possible to compile and test against gevent installed in development
mode via `pip install -e` because pxd files are there in gevent worktree
and tarball):

                      gevent-1.5   gevent-1.5
                      (py import)  (cimport)

    name              old time/op  new time/op  delta
    pyx_select_nogil  8.74µs ± 0%  7.90µs ± 1%  -9.60%  (p=0.000 n=9+10)
    pyx_go_nogil      12.0µs ± 1%  11.2µs ± 2%  -6.35%  (p=0.000 n=10+10)
    pyx_chan_nogil    6.32µs ± 1%  5.89µs ± 0%  -6.80%  (p=0.000 n=10+9)
    go                13.4µs ± 1%  12.4µs ± 1%  -7.54%  (p=0.000 n=10+9)
    chan              6.79µs ± 0%  6.42µs ± 0%  -5.47%  (p=0.000 n=10+10)
    select            10.0µs ± 1%   9.4µs ± 1%  -6.39%  (p=0.000 n=10+10)

but we cannot use cimport to access gevent-1.5 universally, since pxd are not
shipped in gevent wheel releases.

In the future we might want to change plain version check into compile time
check whether gevent/_greenlet.pxd is actually present or not and use faster
access if yes. Requesting gevent to be installed in non-binary form
might be also an option worth trying.

However plain version check should be ok for now.

[1] https://github.com/gevent/gevent/issues/1568

on i7@2.6GHz it looks like:

thread runtime:

    name              time/op
    pyx_select_nogil  2.70µs ±13%
    pyx_go_nogil      15.9µs ± 1%
    pyx_chan_nogil    2.79µs ± 2%
    go                17.6µs ± 0%
    chan              3.05µs ± 4%
    select            3.62µs ± 4%

gevent runtime (gevent-1.4.0):

    name              time/op
    pyx_select_nogil  9.39µs ± 1%
    pyx_go_nogil      15.1µs ± 2%
    pyx_chan_nogil    7.10µs ± 1%
    go                16.6µs ± 1%
    chan              7.47µs ± 1%
    select            10.7µs ± 0%

Just a single change to expose time constants to C++ users (2476f47e).

This makes them available for C++ users as well.

On macos and windows, Python2 is built with --enable-unicode=ucs2, which
makes it to use UTF-16 encoding for unicode characters, and so for
characters higher than U+10000 it uses surrogate encoding with _2_
unicode points, for example:

        >>> import sys
        >>> sys.maxunicode
        65535                       <-- NOTE indicates UCS2 build
        >>> s = u'\U00012345'
        >>> s
        u'\U00012345'
        >>> s.encode('utf-8')
        '\xf0\x92\x8d\x85'
        >>> len(s)
        2                           <-- NOTE _not_ 1
        >>> s[0]
        u'\ud808'
        >>> s[1]
        u'\udf45'

This leads to e.g. b tests failing for

    # tbytes                        tunicode
    (b"\xf0\x90\x8c\xbc",           u'\U0001033c'),     # Valid 4 Octet Sequence '𐌼'

    >           assert b(tunicode) == tbytes
    E           AssertionError: assert '\xed\xa0\x80\xed\xbc\xbc' == '\xf0\x90\x8c\xbc'
    E             - \xed\xa0\x80\xed\xbc\xbc
    E             + \xf0\x90\x8c\xbc

because on UCS2 python build u'\U0001033c' is represented as 2 unicode
points:

    >>> s = u'\U0001033c'
    >>> len(s)
    2
    >>> s[0]
    u'\ud800'
    >>> s[1]
    u'\udf3c'
    >>> s[0].encode('utf-8')
    '\xed\xa0\x80'
    >>> s[1].encode('utf-8')
    '\xed\xbc\xbc'

-> Fix it by detecting UCS2 build and working around by manually
combining such surrogate unicode pairs appropriately.

A reference on the subject:

https://matthew-brett.github.io/pydagogue/python_unicode.html#utf-16-ucs2-builds-of-python-and-32-bit-unicode-code-points

This is a preparatory step for the next patch where we'll be fixing
strconv for Python2 builds with --enable-unicode=ucs2, where a unicode
character can be taking _2_ unicode points.

In that general case relying on unicode objects to represent runes is
not good, because many things generally do not work for U+10000 and
above, e.g. ord breaks:

    >>> import sys
    >>> sys.maxunicode
    65535                       <-- NOTE indicates UCS2 build
    >>> s = u'\U00012345'
    >>> s
    u'\U00012345'
    >>> s.encode('utf-8')
    '\xf0\x92\x8d\x85'
    >>> len(s)
    2                           <-- NOTE _not_ 1
    >>> ord(s)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: ord() expected a character, but string of length 2 found

so we switch to represent runes as integer, similarly to what Go does.

Commit 8af78fc5 (pyx.build: v↑ setuptools_dso (1.2 -> 1.4)) upgraded
setuptools_dso to 1.4, but since from

https://github.com/mdavidsaver/setuptools_dso/commit/3f3ff746

setuptools_dso started to use multiprocessing, pyx.build, when running
under gpython, started to hang, which is a known gevent problem - see
e.g. here: https://github.com/gevent/gevent/issues/993. The problem was
manifesting itself as pyx.build unit test hanging under Python3.

Fix it by installing gevent multiprocessing plugin which is
automatically used/activated by gevent.monkey.patch_all().

geventmp says it is pre-alpha, but by using it we can unhang pyx.build
tests, which is better state than before. The other future possibility
would be to use https://github.com/jgehrcke/gipc wrapped into
multiprocessing compatible API.

This is top-level documentation for error chaining that was promised
and marked as TODO in

- fd95c88a (golang, errors, fmt: Error chaining (C++/Pyx))
- 17798442 (golang: Expose error at Py level)
- 78d0c76f (golang: Teach pyerror to be a base class)
- 337de0d7 (golang, errors, fmt: Error chaining (Python))
- 03f88c0b (errors: Take .__cause__ into account)

This provides top-level documentation for b and u that was promised and
marked as TODO in bcb95cd5 (golang: Provide b, u for strings).

Pychan provides __eq__ (see 2c8063f4 "*: Channels must be compared by
==, not by "is" even for nilchan"), but does not provide __ne__. At the
same time in 17798442 (golang: Expose error at Py level) we had to
define both pyerror.__eq__ and pyerror.__ne__ because without __ne__
pyerror != pyerror was not working correctly.

As it turns out pychan != pychan already works ok, because pychan does
not have base class and for that case cython automatically generates
__ne__ based on __eq__:

    https://github.com/cython/cython/blob/0.29.14-629-ga73815042/Cython/Compiler/ModuleNode.py#L1963-L1976
    https://github.com/cython/cython/commit/b75d2942afab

Add corresponding comment and extend tests to make sure it is indeed so.

Go version does not provide this, but the topic of sync.RWMutex
downgrading was raised up several times, at least

	https://github.com/golang/go/issues/4026
	https://github.com/golang/go/issues/23513
	https://groups.google.com/forum/#!topic/golang-nuts/MmIDUzl8HA0
	...

Atomic downgrading is often useful to avoid race window in between
Unlock and RLock and, as consequence, having the need to recheck things
after RLock.

We can put this complexity and logic into well-defined RWMutex primitive
instead of throwing it to be solved by every RWMutex user.

Provide sync.RWMutex that can be useful for cases when there are
multiple simultaneous readers and more seldom writer(s).

This implements readers-writer mutex with preference for writers
similarly to Go version.

Only io.EOF and io.ErrUnexpectedEOF for now.
Moved here from wcfs from wendelin.core.

A Python error can have links to other errors by means of both .Unwrap()
and .__cause__ . These ways are both explicit and so should be treated
by e.g. errors.Is as present in error's error chain.

It is a bit unclear, at least initially, how to linearise and order
error chain traversal in divergence points - for exception objects where
both .Unwrap() and .__cause__ are !None. However more closer look
suggests linearisation rule to traverse into .__cause__ after going
through .Unwrap() part - please see details in documentation added into
_error.pyx

-> Teach errors.Is to do this traversal, and this way now e.g. exception
raised as

	raise X from Y

will be treated by errors.Is as being both X and Y, even if any of X or Y
also has its own error chain via .Unwrap().

Top-level documentation is TODO.

Following errors model in Go and fd95c88a (golang, errors, fmt: Error
chaining (C++/Pyx)) let's add support at Python-level for errors to wrap
each other and to be inspected/unwrapped:

- an error can additionally provide way to unwrap itself, if it
  provides .Unwrap() method. .__cause__ is not taken into account yet,
  but will be in a follow-up patch;
- errors.Is(err) tests whether an item in error's chain matches target;
- `fmt.Errorf("... : %w", ... err)` is similar to `"... : %s" % (..., err)`
  but resulting error, when unwrapped, will return err.
- errors.Unwrap is not exposed as chaining through both .Unwrap() and
  .__cause__ will need more than just "current element" as unwrapping
  state (i.e. errors.Unwrap API is insufficient - see next patch), and
  in practice users of errors.Unwrap() are very seldom.

Support for error chaining through .__cause__ will follow in the next
patch.

Top-level documentation is TODO.

See https://blog.golang.org/go1.13-errors for error chaining overview.

It is surprising to have an exception class that cannot be derived from.

Besides, in the future we'll use subclassing from golang.error as an
indicator that an error is a "well-defined" (in simple words - does not
need traceback to be interpreted).