TRACKER: support for Python 3.15

[jorisvandenbossche]

We probably already support Python 3.15 in practice since CI is passing (with only one real fix in the datetime code), but there a few more things to do, so tracking here:

• Enable in CI: CI: enable python3.15 job #66473
• Enable building py 3.15 wheels:
  • cibuildwheel update that includes support: Bump pypa/cibuildwheel from 4.1.1 to 4.2.0 #66733
  • include py3.15 in the wheel matrix (and upload nightly wheels)
• Indicate support in pyproject.toml and release notes

xref #62261 for Python 3.14