feat: support Python 3.15

[PyYoshi]

Summary

• test CPython 3.15 and free-threaded 3.15t on Linux, macOS, and Windows
• build cp315 and cp315t wheels and add the Python 3.15 classifier
• bump cibuildwheel to v4.3.0, which builds and smoke-tests wheels against CPython 3.15.0rc3 (ABI-compatible with the final release, scheduled for 2026-10-09)
• update dependencies to their latest releases:
  • uv lock --upgrade: ast-serialize 0.12.1, charset-normalizer 3.5.2, cryptography 50.0.2, idna 3.20, librt 0.16.0, mypy 2.4.0, pyproject-hooks 1.3.3, ruff 0.16.10, urllib3 2.8.0, zipp 4.1.1
  • raise the dev dependency-group lower bounds to the locked versions
  • astral-sh/setup-uv v10.1.0 → v10.2.0 (SHA-pinned), actions/download-artifact v7 → v8

The extension needs no source changes: the Cython output compiles against the 3.15 headers without deprecation warnings, and the existing freethreading_compatible directive keeps the GIL disabled on 3.15t. The build-system requirement Cython>=3.2.9,<3.4 is intentionally unchanged because Cython 3.2.9 also builds and passes the tests on 3.15 and 3.15t. Isolated builds pick Cython 3.3.0, which includes the upstream 3.15 adaptations.

actions/download-artifact v8 errors on digest mismatches by default and no longer unzips non-zip downloads. The tag-only publish job downloads only the zipped artifacts uploaded by upload-artifact@v7, so neither change affects it. That job does not run on pull requests.

Validation

• 29 tests pass on CPython 3.11, 3.14, 3.15.0rc3, and free-threaded 3.15.0rc3t using the CI steps (uv sync --locked then uv run pytest)
• free-threaded 3.15.0rc3t imports with -W error and leaves sys._is_gil_enabled() false
• Ruff (check and format) and strict mypy pass with the upgraded tools
• with cibuildwheel 4.3.0, real cp315-manylinux_x86_64 and cp315t-manylinux_x86_64 wheels build, repair, install, and pass tools/wheel_smoke.py
• cibuildwheel 4.3.0 selects cp315/cp315t identifiers for Linux x86_64/aarch64, macOS x86_64/arm64, and Windows win32/amd64
• the upgraded uv.lock (lock revision 5) passes lock --check and sync --locked with uv 0.9.26, the required-version minimum, as well as with uv 0.12.23

References

• PEP 790 (Python 3.15 release schedule): https://peps.python.org/pep-0790/
• cibuildwheel v4.2.0 (cp315 built by default): https://github.com/pypa/cibuildwheel/releases/tag/v4.2.0
• cibuildwheel v4.3.0: https://github.com/pypa/cibuildwheel/releases/tag/v4.3.0
• Cython 3.3.0 changelog (Python 3.15 adaptations): https://github.com/cython/cython/blob/master/CHANGES.rst
• actions/download-artifact v8.0.0: https://github.com/actions/download-artifact/releases/tag/v8.0.0