Move from deprecated find_loader() to find_spec() for setting up the import time package path (GH-4764)

Closes https://github.com/cython/cython/issues/4763

This reverts commit 74073417.

This reverts commit 74073417.

Move from deprecated find_loader() to find_spec() for setting up the import time package path (GH-4764)

Closes https://github.com/cython/cython/issues/4763

They prevent explicitly calling the base-class __contains__.

Closes https://github.com/cython/cython/issues/4785

Fix version detection and compile/runtime Python version warning with double-digit versions such as Py3.10 (GH-4777)

See https://github.com/cython/cython/issues/2542#issuecomment-1121484721

Closes https://github.com/cython/cython/issues/4637
See See https://github.com/cython/cython/issues/3556

* 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.

See https://github.com/cython/cython/issues/3554

* Make the GIL-avoidance in 7d99b0f0 actually work in nogil functions and not just nogil sections.

See https://github.com/cython/cython/issues/3558

This means that reverse operators (e.g. `__radd__`) won't be
hidden by the automatic wrapper that `PyType_Ready()` produces if
the forward method exists. Although they won't work as
in Python, they will be possible to look up and call explicitly.
This should make it easier to write code that's compatible with
Cython 0.29.x and Cython 3 (where reverse operators will be full supported).

Closes https://github.com/cython/cython/issues/4750

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.

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 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/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

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

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

Make the pickle test actually test that auto-pickling uses the right checksum (and not a user implemented reduce method that uses no checksums at all).

Make error message more readable by using hex integer values for all presented checksums, not just the compile time ones.

For the auto-pickle checksum, allow SHA-1 and SHA-256 which are used by Cython 3.x pickles now. Otherwise stick to MD5 since that was used before.

Closes https://github.com/cython/cython/issues/4680

In https://github.com/python/cpython/pull/31530
https://bugs.python.org/issue46836

the `_frame` struct was moved to an internal header, however the public API is
primarily read-only, and Cython needs to build PyFrameObjects so still import
the internal headers.

Also sets the Py_BUILD_CORE define for py311a6, trying to restrict it to the frame header.

This creates two copies of the function, one for the exception
case and one for the non-exception case. It's probably inefficient
but the simplest solution.

Fixes https://github.com/cython/cython/issues/4651.