Don't automatically add PyInit_<name> to export symbols in Python 3.15+

[encukou]

Since PEP 793, it's valid for modules to not have a PyInit function. Telling MSVC to export a nonexistent function will make it fail.

The function should be exported in code, using PyMODINIT_FUNC (which adds __declspec(dllexport), which is preferred over /EXPORT according to Microsoft docs.)

Fixes: #387

[rgommers]

This implementation looks good to me, for now I only have a few very small suggestions for the docstring. I still need to do some testing with a few real-world packages.

For other reviewers: this relies on python/cpython#141672, which was merged only last week.

[rgommers]

The original could possibly confuse the reader, since it could be read as
the package author having to do something - so try to rephrase:

Suggested change

	An export directive should be in the code itself.
	An export directive was added to the "PyMODINIT_FUNC" implementation
	for Python 3.15, which is all that is needed.

[rgommers]

Suggested change

	it adds "PyInit_" + module_name 'ext.export_symbols'.
	it adds "PyInit_" + module_name to 'ext.export_symbols'.

[rgommers]

I tested this change with the setuptools/c test case in https://github.com/Quansight-Labs/stable-abi-testing (side note: not yet fully polished, but it's usable enough to derive test cases from now; it contains test packages in a build-backends x binding-generators matrix).

With the latest released version of setuptools, that test package fails to build with 3.15.0a3 - which is the most recent build available in GHA's setup-python - with (CI log):

LINK : error LNK2001: unresolved external symbol PyInit_limited

After applying the patch in this PR to setuptools._distutils, that error goes away and the package builds and passes its tests (CI log).

That repo uses test packages with PyMODEXPORT_FUNC rather than PyMODINIT_FUNC, so that does not yet prove that this also works with the latter.

I then started with psutil, tested the patch with its existing wheel build CI matrix for completeness - that passed of course, since 3.15 wheels are not included yet (no cibuildwheel support), and this PR is a no-op on older Python versions.

I then also added a new psutil CI job for 3.15, before figuring out that psutil depends on pywin32 which doesn't have an sdist and doesn't build from source. So that's where I gave up.

Testing on 3.15-alpha is pretty cumbersome given the many packaging components and packages that don't yet support 3.15. That said, I'm confident that this PR works, and the testing I was able to do all worked as expected - so I'm +1 for merging this PR.