Wheels for Python 3.15 and 3.15t #874

Python 3.15.0 is out, but numcodecs 0.17.0 has no cp315 or cp315t wheels on PyPI, so pip install numcodecs on 3.15 builds from source. #842 (start testing Python 3.15) was closed without being merged, and I couldn't find another issue tracking this.

The wheel build currently uses CIBW_BUILD: "cp312-* cp313-* cp314-* cp314t-*" in .github/workflows/wheel.yaml; adding cp315-* and cp315t-* there (plus 3.15 in the test matrix) would probably be most of it.