Add Python 3.15 support

[ml31415]

Python 3.15.0 was released today (2026-10-09). This adds it everywhere a supported Python version is declared:

• .github/workflows/ci.yaml: 3.15 added to the test matrix.
• .github/workflows/pypi-release.yaml: build/test leg of the release workflow bumped from 3.12 to 3.15, so releases are built and smoke-tested on the newest Python.
• pyproject.toml: Programming Language :: Python :: 3.15 classifier added. The PyPI pyversions badge in the README follows from these classifiers, so no README change is needed (the benchmark table keeps its documented provenance: those numbers were measured on the maintainers' i7-7560U machine with Python 3.14).

Validation against CPython 3.15.0 (final, built from the python.org release tarball)

• Dependency stack resolves on 3.15 via the existing universal uv.lock (unchanged): numpy 2.5.3, numba 0.68.0, pandas 3.0.6, pytest 9.1.1 — all ship cp315 wheels.
• Full test suite: 2396 passed, 174 skipped, 924 deselected — identical to the 3.14.7 baseline run on the same tree (same skip/deselect pattern).
• Release flow reproduced locally: uv build → twine check --strict (PASSED) → sdist installed into a fresh 3.15 venv → import and functional smoke OK.
• Benchmark module smoke run on 3.15 OK (Numba JIT compilation works).

Side note (no action taken): Python 3.10 reached end-of-life on 2026-10-01; dropping it from the matrix and requires-python can be a separate decision.