Python free-threading support

[Avasam]

Windows-specific

• pywin32 (no sdist) Experimental Free-Threaded support mhammond/pywin32#2303
  • source builds available but probably broken at runtime: Third try at adding no-GIL support mhammond/pywin32#2767

All OSes

• PySide6-Essentials (no sdist) https://qt-project.atlassian.net/browse/PYSIDE-2221
  • Can't even fake a threading wheel. Because of the lack of abi feature environment marker, I can't get only the free-threading build to use a custom wheel. I'd need to fake a local simple source with all the necessary wheels.
    • https://peps.python.org/pep-0780/
    • https://discuss.python.org/t/pep-780-abi-features-as-environment-markers/86013
    • Add sys.abi_info to make information about the interpreter ABI more accessible python/cpython#133143
• opencv-contrib-python-headless (failed sdist build) Python "No GIL" aka "Free Thread" opencv/opencv#27933 (comment) & [Question] Python 3.13t (free-threaded) compatibility opencv/opencv-python#1029
  • In theory I could just build from source, but these two issues are making it harder:
    • Missing sdists on PyPI opencv/opencv-python#1213
    • Add CI support for building Windows on ARM wheels opencv/opencv-python#1143
    • Disable limited API for free-threaded Python opencv/opencv-python#1051