This continues 6b4990f6 (gpython: Fix pymain to properly setup
sys.path[0]) - now we add test and verify that gpython actually sets
sys.path in the same way as underlying python does:

- don't add directory of gpython to sys.path
- for `gpython file` add to sys.path[0] realpath(dir(file)) instead of
  dir(file)  (see PySys_SetArgvEx for the reference, and else - if we do
  not make this change - added test breaks)
- for `gpython -m mod` add to sys.path[0] realpath('') instead of ''.
  This exactly matches PY3 behaviour, while PY2 behaviour is to add ''
  However PY3 behaviour

    * is more sane, and
    * fixes test_defer_excchain_traceback on py27-gevent

  -> so we stick to it.

Now all tests pass again.

GPython preimports golang and gevent, but until now, it was preimporting
them before adjusting sys.path according to given -c/-m/file/... And
this was leading to difference in behaviour with standard Python where
e.g. `import golang` might result in different files loaded if e.g.
golang/ is there on cwd and -c/-m was given.

-> Rework gpython startup so that golang/gevent preimport happens after
sys.path initialization.

This fixes `tox -e py27-gevent` which was previously failing on
test_defer_excchain_dump_pytest / test_defer_excchain_dump_ipython
because dumped paths were different than expected ones.

It, however, breaks test_defer_excchain_traceback on py27-gevent.  ->
We'll do more fixups to sys.path handling in the next patch which will
fix that failure as well.

We'll need this in the next patch, where golang/gevent preimport will be
moved from "before call to pymain", to "in between option parsing and
interpreter start".

We'll need warnings control in the next patch to fix Pytest integration
wrt python3-dbg with `-W ignore::DeprecationWarning`.

Reorganize parsiong of command line options so that first all options are
parsed in a loop, and only after that a module/file/command is executed.

This is needed as preparatory step for next patch: there we'll add
support for -W, and `-W arg` can be given multiple times and has to be
processed multiple times by creating multiple corresponding warning
filters. Because those warning filters has to be applied uniformly to
all 4 codepaths of execution phase (-m/-c/file/interactive console), it
makes sense to move execution phase to after options parsing and inject
common runtime preparatory steps right before that.

This logic generally applies not only to -W, but to all other python
options - e.g. -Q,-u,...

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 !5

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

/reviewed-on !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 !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)

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: !4

The first step to expose errors and error chaining to Python:

- Add pyerror that wraps a pyx/nogil C-level error and is exposed as golang.error at py level.
- py errors must be compared by ==, not by "is"
- Add (py) errors.New to create a new error from text.
- a C-level error that has .Unwrap, is exposed with .Unwrap at py level,
  but full py-level chaining will be implemented in a follow-up patch.
- py error does not support inheritance yet.

Top-level documentation is TODO.

With Python3 I've got tired to constantly use .encode() and .decode();
getting exception if original argument was unicode on e.g. b.decode();
getting exception on raw bytes that are invalid UTF-8, not being able to
use bytes literal with non-ASCII characters, etc.

So instead of this pain provide two functions that make sure an object
is either bytes or unicode:

- b converts str/unicode/bytes s to UTF-8 encoded bytestring.

	Bytes input is preserved as-is:

	   b(bytes_input) == bytes_input

	Unicode input is UTF-8 encoded. The encoding always succeeds.
	b is reverse operation to u - the following invariant is always true:

	   b(u(bytes_input)) == bytes_input

- u converts str/unicode/bytes s to unicode string.

	Unicode input is preserved as-is:

	   u(unicode_input) == unicode_input

	Bytes input is UTF-8 decoded. The decoding always succeeds and input
	information is not lost: non-valid UTF-8 bytes are decoded into
	surrogate codes ranging from U+DC80 to U+DCFF.
	u is reverse operation to b - the following invariant is always true:

	   u(b(unicode_input)) == unicode_input

NOTE: encoding _and_ decoding *never* fail nor loose information. This
is achieved by using 'surrogateescape' error handler on Python3, and
providing manual fallback that behaves the same way on Python2.

The naming is chosen with the idea so that b(something) resembles
b"something", and u(something) resembles u"something".

This, even being only a part of strings solution discussed in [1],
should help handle byte- and unicode- strings in more robust and
distraction free way.

Top-level documentation is TODO.

[1] zodbtools!13

sys is imported in the beginning of main, so there is no need to import
it the second time in the middle of main. The bug was there since
gpython day1 - since 32a21d5b (gpython: Python interpreter with support
for lightweight threads).

pyrun runs command without piping output.
pyout runs command and returns its output.

We'll need to use this utility in other places; golang_test is the
central place for testing utilities.

gpython/gpython_test.py:23: 'subprocess' imported but unused
gpython/gpython_test.py:24: 'six.PY3' imported but unused

- we are going to introduce golang.time, and from inside there without
  `from __future__ import absolute_imports` it won't be possible to import
  needed stdlib's time.

- we were already doing `from __future__ import print_function`, but
  only in some files.

-> It makes sense to apply updated __future__ usage uniformly.

	pypy-gevent runtests: commands[0] | gpython -m pytest gpython/ golang/
	Traceback (most recent call last):
	  File "/home/kirr/src/tools/go/pygolang/.tox/pypy-gevent/bin/gpython", line 10, in <module>
	    sys.exit(main())
	  File "/home/kirr/src/tools/go/pygolang/.tox/pypy-gevent/site-packages/gpython/__init__.py", line 153, in main
	    _ = monkey.patch_all()      # XXX sys=True ?
	  File "/home/kirr/src/tools/go/pygolang/.tox/pypy-gevent/site-packages/gevent/monkey.py", line 976, in patch_all
	    patch_thread(Event=Event, _warnings=_warnings)
	  File "/home/kirr/src/tools/go/pygolang/.tox/pypy-gevent/site-packages/gevent/monkey.py", line 178, in ignores
	    return func(*args, **kwargs)
	  File "/home/kirr/src/tools/go/pygolang/.tox/pypy-gevent/site-packages/gevent/monkey.py", line 588, in patch_thread
	    _patch_existing_locks(threading_mod)
	  File "/home/kirr/src/tools/go/pygolang/.tox/pypy-gevent/site-packages/gevent/monkey.py", line 492, in _patch_existing_locks
	    if o._RLock__owner is not None:
	AttributeError: 'thread.RLock' object has no attribute '_RLock__owner'

It will be fixed on next Gevent release, however until then the crash is there
and we have to workaround: we can skip patching existing locks as it will be
the same behaviour of next Gevent release, since its just not possible to patch
locks created via instantiated C-level classes:

https://github.com/gevent/gevent/commit/d0e04658

We are doing monkey-patching very early, so it should be safe.

PyPy7 always pre-imports the time module. Without the change tests were
failing:

	Traceback (most recent call last):
	  File "/home/kirr/src/tools/go/pygolang/.tox/pypy3-gevent/bin/gpython", line 10, in <module>
	    sys.exit(main())
	  File "/home/kirr/src/tools/go/pygolang/.tox/pypy3-gevent/site-packages/gpython/__init__.py", line 145, in main
	    '\n\n\t%s\n\nsys.modules:\n\n\t%s' % (bad, sysmodv))
	RuntimeError: gpython: internal error: the following modules are pre-imported, but must be not:

	        ['time']

	sys.modules:

	        ['__future__', '__main__', '__pypy__', '__pypy__._pypydatetime', '__pypy__.builders', '__pypy__.intop', '__pypy__.os', '__pypy__.thread', '__pypy__.time', '_ast', '_bootlocale', '_codecs', '_collections', '_collections_abc', '_continuation', '_csv', '_frozen_importlib', '_frozen_importlib_external', '_imp', '_io', '_locale', '_multibytecodec', '_operator', '_rawffi', '_rawffi.alt', '_signal', '_sre', '_structseq', '_thread', '_warnings', '_weakref', '_weakrefset', 'abc', 'array', 'builtins', 'codecs', 'copyreg', 'encodings', 'encodings.aliases', 'encodings.ascii', 'encodings.latin_1', 'encodings.utf_8', 'errno', 'gc', 'genericpath', 'gpython', 'marshal', 'os', 'os.path', 'posix', 'posixpath', 'pwd', 're', 'site', 'sre_compile', 'sre_constants', 'sre_parse', 'stat', 'sys', 'time', 'unicodedata']

Pymain was not adding e.g. directory of executed file to sys.path, and
as the result if there were e.g. 2 files

	dir/hello.py	# imports world
	dir/world.py

running `gpython dir/hello.py` would fail to import world.

The case for interactive console was also failing to setup sys.argv as
empty, so it was containing ['/path/to/gpython']. It was also, contrary
to standard python, unconditionally showing '>>>' prompt even when stdin
was not a tty.

Fix all that and add a test to cover pymain functionality.

Provide gpython interpreter, that sets UTF-8 as default encoding,
integrates gevent and puts go, chan, select etc into builtin namespace.
Please see details in the documentation added by this patch.

pymain was taken from kirr/go123@7a476082
(go123: tracing/pyruntraced: New tool to run Python code with tracepoints activated (draft))