Support Python 3.13

[kmaehashi]

Python 3.13 is out. We will likely need to bump to Cython 3.1.0+ to support running under the Free-threaded mode. Ref: https://py-free-threading.github.io/tracking/

Tasks

• Read Release Notes
• Check fastrlock compatibility Build failing with Python 3.13 scoder/fastrlock#20
• Install Python to Windows CI image (ask @kmaehashi)
• Fix code and CI to support the new vesrion. Add CI for Python 3.13 and mpi4py v4 #8953
  • FlexCI: Update .pfnci/schema.yaml and .pfnci/matirx.yaml (note: Some Python libraries may not be supported with the newer versions of Python. In that case you should also limit the library version.)
  • Package Metadata: Update package classifier and python_requires tag (if needed) in setup.py.
• Backport the above PR. [backport] Add CI for Python 3.13 and mpi4py v4 #8974
• Fix cupy-release-tools to support the new version. Add support for CUDA 12.8 and Python 3.13 cupy-release-tools#397
• Backport the above PR. [backport] Add support for CUDA 12.8 and Python 3.13 cupy-release-tools#398
• Update matrix configuration in cupy-package-builder (internal tool)
• Fix documentation. Update list of supported versions #8986
  • Update Requirements section in the Installation Guide
• Backport the above PR.

[hauntsaninja]

fastrlock 0.8.3 now supports Python 3.13! Would be nice to just have wheels for Python 3.13 for cupy-cuda (regular build, not free threading)

[jakirkham]

As Leo noted, upgrading to Cython 3 ( #4610 ) will also be needed for Python 3.13 support generally

This because Cython 0.29 lacks Python 3.13 support

xref: #8457 (comment)