Only #define PY_SSIZE_T_CLEAN if it is not already #defined

Signed-off-by: Andrew V. Jones <andrew.jones@vector.com>

* Reject invalid underscore placements in float parser.
* Add a proper nan/inf float parser to prevent underscore-mixes like "in_f" from passing through.

Make a helper function in a C++ test correctly propagate exceptions so that it won't have to spit out compiler warnings.

Special case float parsing in PyPy since byte string access is likely slower there than a plain float object creation. (And it works in all PyPy versions.)

* DOC: make some github issues into links, doc fixes
* add a doc-requirements.txt for building docs
* use ':issue:' domains for github issues

Fixes https://github.com/cython/cython/issues/3537

Both the C code and the tests are largely copied from CPython.

Note that this currently only applies to Python classes, not to cdef classes. Maybe it could be applied to the second+subsequent bases but I don't think it's needed for the initial implementation.

Freeze the test dependency packages for Python 2.7 to prevent further deprecations. More and more packages drop Py2 support these days.

Fix include file: "pytime.h" is actually included by "Python.h" and should not get included directly.

Fix MSVC: Add an explicit warning to modules that use the "array.array" internals and try to compile in PyPy.
Closes https://github.com/cython/cython/issues/3448

The __self__ argument should be present only for bound functions.
Fixes https://github.com/cython/cython/issues/4036

Currently it isn't easily possible to get this working absolutely correctly for fused functions. I raise an attribute error but hasattr still returns True.

Additionally, Python 2 methods always have a __self__ attribute but set it to None. I follow Python 3 behaviour and don't have the attribute.

Closes https://github.com/cython/cython/issues/3448

Remove GNU specific "tm" struct fields "tm_zone" and "tm_gmtoff" from libc/time.pxd because they get in the way of automatic struct conversions.
See https://github.com/cython/cython/issues/3733

See https://github.com/cython/cython/issues/3919

The loop parameter of asyncio.sleep(), deprecated in Python 3.8, has
been removed in Python 3.10. asyncio.sleep() gets implicitly the loop
in Python 3.10 and it is safe to do the same on Python 3.6:
https://docs.python.org/dev/whatsnew/3.10.html#changes-in-the-python-api

See https://github.com/cython/cython/issues/3919

Backport code from the master branch:

* commit 4e785d42
* commit a9fb7768

Avoid calling "PyErr_Clear()" in "__Pyx_CyFunction_get_is_coroutine()" if no error was raised (by our code).

* Avoid UtilityCode from cimport_from_pyx files
* Unique names for generated cfunc wrappers, avoid pxd pickle code
* Set .in_pxd on nodes prior to analyse_declarations

Fixes #3716
Fixes #3741
Fixes #3734

Mostly with the hope that the tests pass. Ideally we should support the most recent version.

Closes https://github.com/cython/cython/issues/2233

Even though the Python documentation states, that PyUnicode_KIND will return an int,
it actually returns a value of the underlying type of the PyUnicode_Kind enum.
This could be e.g. an unsigned int which can cause warnings about signed/unsigned comparisons.

Backported from 3dc2b9df
Partial fix for https://github.com/cython/cython/issues/3919

clang warns about dead code when an if-statement always evaluates to false. This warning can be silenced using an extra '()'.