ci: build Python 3.15 wheels (stacked on #229)

[iDoMeteor]

Summary

Build Python 3.15 wheels, as requested by @shauneccles on #229.

This PR is stacked on #229 (Python 3.14 wheels, by @cpmpercussion) and contains
one commit on top of it, ci: build Python 3.15 wheels. The diff is only meaningful
once #229 is in: the first commit shown here is #229's. If #229 merges first, this
rebases to a single commit with no conflicts.

Changes (pyproject.toml, tox.ini only):

• add cp315 to the [tool.cibuildwheel.linux], .macos and .windows build patterns
• add the Programming Language :: Python :: 3.15 classifier
• add py314 and py315 to the tox.ini envlist (py314 was missing after ci: build Python 3.14 wheels #229; with
  skip_missing_interpreters = True this is harmless where an interpreter is absent)

No workflow changes are needed, and no cpython-prerelease opt-in: #229 already moves
cibuildwheel to v4.2.1, and cibuildwheel 4.2.0 and later build CPython 3.15 by default
("CPython 3.15 wheels are now built by default - without the cpython-prerelease
enable set"; v4.2.1 bundles 3.15.0rc2).

Python 3.15 status (as of 2026-10-05)

• 3.15.0rc3 was released 2026-10-02; 3.15.0 final is scheduled for 2026-10-09 (PEP 790).
• The ABI has been frozen since 3.15.0rc1 (2026-08-04). The release notes say there will be
  no ABI changes from that point on and that wheels built against 3.15 release candidates
  will work with later 3.15 releases, so the wheels are safe to upload now and stay valid
  for the final release.
• Tooling that the build resolves: Cython 3.3.0 ("changes were made to adapt to Python 3.15",
  and the cpython.array declarations were adapted for 3.15), meson-python 0.22.1, meson 1.12.1.
  pyproject.toml leaves cython unpinned, so a build today gets 3.3.0. trove-classifiers
  already knows the 3.15 classifier.

Sources: peps.python.org/pep-0790, python.org release pages for 3.15.0rc1 and rc3,
cibuildwheel changelog (v4.2.0, v4.2.1, v4.3.0), Cython CHANGES.rst for 3.3.0,
meson-python CHANGELOG.rst.

Verification

This repo's PR workflow, on GitHub Actions. pr_to_master.yml was run unchanged against this branch from a separate repo; only the checkout target and the artifact names differed. Every job passed: sdist, ubuntu-latest, ubuntu-24.04-arm, macos-latest and windows-latest, with cibuildwheel's tests run as configured. It built cp315 wheels for manylinux_x86_64, manylinux_aarch64, macosx_10_15_x86_64, macosx_11_0_arm64 and win_amd64, alongside #229's cp314 ones (41 wheels in total).

Also done locally (Linux x86_64, Fedora host):

• cibuildwheel --print-build-identifiers (4.2.1) with this pyproject.toml selects cp315
  on all three platforms, in addition to everything ci: build Python 3.14 wheels #229 selects:

  • linux: cp315-manylinux_x86_64
  • macos: cp315-macosx_x86_64, cp315-macosx_arm64
  • windows: cp315-win_amd64

  It selects no cp315t (free-threaded) identifiers.

• Built and tested with cibuildwheel 4.2.1 under podman, using the configured before-all,
  repair-wheel-command and test-command (pytest -v -m ci {package}/tests):

  • cp315-manylinux_x86_64 -> python_rtmidi-1.6.0-cp315-cp315-manylinux_2_24_x86_64.manylinux_2_28_x86_64.whl
    (CPython 3.15.0rc2 in the image; 8 passed, 2 skipped)
  • cp314-manylinux_x86_64 for comparison -> the matching cp314 wheel (8 passed, 2 skipped)

• Built and installed from source on a python-build-standalone CPython 3.15.0rc3 with
  Cython 3.3.0, meson-python 0.22.1 and meson 1.12.1 (pip install --no-build-isolation .),
  then ran the full test suite against real ALSA: 38 passed, 2 skipped, 1 failed. The failure
  is test_ci_wheels.py::test_linux_supports_jack, expected on a machine without JACK
  (the same test passes in the cibuildwheel container, which builds JACK in before-all).

• pyproject.toml, the three workflow files and tox.ini parse cleanly.

Not verified:

• I did not run tox and did not touch the black target-version list in pyproject.toml
  (it stops at py313, and ci: build Python 3.14 wheels #229 did not extend it for 3.14 either).
• No CHANGELOG.md entry: it only has per-release sections, and ci: build Python 3.14 wheels #229 does not add one.
  Happy to add a line if you want it.

Notes

• cibuildwheel 4.3.0 (released today) updates the bundled CPython to 3.15.0rc3; v4.2.1 is
  fine for the ABI reasons above, so I did not bump it here.
• Free-threaded wheels (cp314t/cp315t) are deliberately not part of this PR; they depend
  on the free-threading support in Support free-threaded Python, and fix instance lifetime and threading bugs #230 and will be proposed separately.

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