Fix 'redefinition; different linkage' errors with cp313-win

[cdce8p]

After updating mypyc to the latest pythoncapi_compat.h, the cp313-win wheel builds started to fail.

  C:\Users\runneradmin\AppData\Local\pypa\cibuildwheel\Cache\nuget-cpython\python.3.13.0\tools\include\internal\pycore_unicodeobject.h(262): error C2375: '_PyUnicode_Equal': redefinition; different linkage
  D:\a\mypy_mypyc-wheels\mypy_mypyc-wheels\mypy\mypyc\lib-rt\pythoncapi_compat.h(1540): note: see declaration of '_PyUnicode_Equal'

  C:\Users\runneradmin\AppData\Local\pypa\cibuildwheel\Cache\nuget-cpython\python.3.13.0\tools\include\internal\pycore_pyhash.h(24): error C2375: '_Py_HashBytes': redefinition; different linkage
  D:\a\mypy_mypyc-wheels\mypy_mypyc-wheels\mypy\mypyc\lib-rt\pythoncapi_compat.h(1567): note: see declaration of '_Py_HashBytes'

https://github.com/mypyc/mypy_mypyc-wheels/actions/runs/11327195039/job/31497783023#step:4:3766

MSVC seems to require that the signature matches the upstream one exactly.

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The fix here worked for mypyc. Not sure if there is a better one.
/CC @vstinner

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Refs:

• Add PyUnicode_Equal() function #110
• Add Py_HashBuffer() function #112
• The mypy PR which broke the wheel builder: [mypyc] Sync pythoncapi_compat.h mypy#17929
• The working fix: [mypyc] Fix wheel build for cp313-win mypy#17941

[vstinner]

Hi, the CI was broken, I just fixed it. Would you mind to update your branch? For example, rebase it on top on the up to date main branch.

[vstinner]

Merged, thank you!