Fixes issue https://github.com/cython/cython/issues/3065
Fixes issue https://github.com/cython/cython/issues/3066

* delete cpp_exception_declaration_compatibility test
  Because I'm emitting an error at an earlier stage I don't think there's a way to get this test to work.

Reported in https://github.com/cython/cython/pull/4228

Closes https://github.com/cython/cython/issues/3922

Partially reverts https://github.com/cython/cython/commit/1027db04fb7c5bbfea2b9bcdfce0b1f384e26a93

Also make a couple of improvements regarding clarity and text formatting.

See https://github.com/cython/cython/issues/4187

* .gitignore: add directory for docs build and cython_debug
* doc-requirements.txt: add sphinx-tabs
* conf.py: add and setup sphinx-tabs extension
* Create _static\css\tabs.css for customisation
* add "two-syntax-variants-used" file as a preface about the different typing variants

See https://github.com/cython/cython/issues/4200Signed-off-by: Stefan Behnel <stefan_ml@behnel.de>

Closes https://github.com/cython/cython/pull/4224

Automatically add stubs for "cython.cimports.*" in the test runner to make them importable in Python (although not necessarily runnable).

Automatically prepend "import cython" to the examples if necessary.

See https://github.com/cython/cython/pull/4213#discussion_r648005583

Closes https://github.com/cython/cython/issues/4214
Closes https://github.com/cython/cython/issues/2811

* Set "entry.qualified_name" for builtin types.
* Make sure we always have a correct "qualified_name" for all symtab Entry instances.

Looking at the Cython code I spotted a few areas where it contains special paths for Pyston v1.
Pyston v1 only supports python2 and is dead but we did release a new Pyston v2 which is a fork of CPython 3.8.
As we don't currently set `PYSTON_VERSION` in v2 the current code can't trigger and can be removed.

I hoped we would not need any special casing for v2 because it should be fully compatible with CPython but
https://github.com/cython/cython/issues/4200 may temporarily need one but I think the current
code is overkill and if necessary I can add (part of) it back later on.

Split the interface of "cythonrun" and "BuildExecutable" so that the latter no longer automatically runs the program.

MSVC appears not like like `#if/#else` with an macro call.
Fixes https://github.com/cython/cython/issues/4202

Fixed optimizations in defined cppclass

The problem was they were being skipped with SkipDeclarations

Assume that all live PyPy3 installations know about the correct Py3 signature of PyEval_EvalCode() by now.
See discussion leading up to https://github.com/conda-forge/cython-feedstock/pull/75#issuecomment-848770641

Implements https://github.com/cython/cython/issues/2090

* Add execution policy overloads to all algorithms
* Add update_cpp17_extension for cpp17 tests
* Fix type deduction logic for forwarding reference argument