This is used by some optimisations for builtins that call C-API functions directly but need to convert None arguments to NULL or special integer values in order to mimic the original Python interface.

Also add and backport the CPython macros for None checks (and True/False, while we're at it):
https://docs.python.org/3/c-api/structures.html#c.Py_Is

Closes https://github.com/cython/cython/issues/4737
See https://github.com/cython/cython/issues/4706

Fixes a bug introduced by https://github.com/cython/cython/commit/ae4ade8daeae206db68583f2f4b9c1cbce9cba75 for PyPy.

PR 4703 was an incomplete backport of the changes needed for #3554 and generates incorrect C code.

See https://github.com/cython/cython/issues/3554
Reverts https://github.com/cython/cython/pull/4703

This reverts commit d395a56f.

Use "\x20" instead of a plain space at the end of a doctest output line to make it easier to edit. Some editors strip off trailing whitespace.

It causes issues while profiling or debugging where global variables can end up inadvertently changed.

Fixes https://github.com/cython/cython/issues/4609

The condition should only evaluate to True when assigning __dict__, but it currently does for _, d, i, etc. as well as resulting in the following potential issues:

* Non-member are being assigned to the object instead of raising.
* The one-field rule can be bypassed.
* Valid members that pass the condition are being assigned raw and are never cast to the specified type.

The condition should only evaluate to True when assigning __dict__, but it currently does for _, d, i, etc. as well as resulting in the following potential issues:

* Non-member are being assigned to the object instead of raising.
* The one-field rule can be bypassed.
* Valid members that pass the condition are being assigned raw and are never cast to the specified type.

The presence of this generic special method should not prevent Cython from making a class pickleable.

Fixes https://github.com/cython/cython/issues/4730

Closes https://github.com/cython/cython/issues/4704

Closes https://github.com/cython/cython/issues/4722

Closes https://github.com/cython/cython/issues/4722

Disable "CYTHON_UPDATE_DESCRIPTOR_DOC" for Pyston. It was previously only enabled for CPython and should only have been enabled additionally for recent PyPy versions.

Stop using "PyBytesObject.ob_shash" in Py3.11, where it is deprecated. It was just an optimisation that isn't very important for byte strings any more.

Closes https://github.com/cython/cython/issues/4721

Add exception handling for `libcpp::stack::push`

Since cimporting directly from "cpython" has long been deprecated.

e.g. https://github.com/cython/cython/issues/4707

Acquire the GIL in nogil functions only when strictly needed on function exit, e.g. for cleaning up temp variables from with-gil blocks or adding tracebacks.

Closes GH-3554
Closes GH-4637

The license follows that of the official Python logo, which it is derived from.

Unconditionally adding `/D_USE_MATH_DEFINES` via cmdline triggers `-Wmacro-redefined` in cython-generated code.

Unconditionally adding `/D_USE_MATH_DEFINES` via cmdline triggers `-Wmacro-redefined` in cython-generated code.

It appears to have been be used for Python < 2.6 (which didn't have the new buffer protocol).
Most of the mechanism had already been removed, but the capsule attributes that it relied on were still generated.

Also removed some utility code that was only used for this feature.