Build and test Python 3.15 wheels

[Mokto]

Summary

Wheels are now built for Python 3.15 on every platform (manylinux, musllinux, Windows, macOS) and the Linux test job runs the suite on both 3.14 and 3.15. The package metadata now lists the 3.15 classifier.

Changes

• Add 3.15 to the interpreter list and to setup-python (with allow-prereleases) in all wheel build jobs.
• Test Linux wheels on a 3.14 and 3.15 matrix, installing only binary wheels so the tests can't fall back to building from source.
• Raise the build requirement to maturin>=1.15.0.
• Run the workflow when python-test.yaml itself changes.

Review focus

Check the test job's uv venv --python "$(command -v python)" change: it pins the venv to the matrix interpreter.