Add support for Python 3.15

[kytta]

Python 3.15 has reached the "release candidate" phase, meaning that the ABI and language APIs are now stable. We can now test under it and declare support for it :)

I ran tests locally; all relevant pass

  py315: OK (4.06 seconds)
  py315t: OK (3.06 seconds)
  lint: OK (1.20 seconds)
  congratulations :) (8.36 seconds)