Test with 3.15 stable (out Oct 1)

Once the stable release is out (Oct 1), add it to our CI.

Start by inspecting the repository's CI configuration and find where supported Python versions are listed. After Python 3.15 is stable, add it to that configuration and run the relevant CI checks; done means the CI workflow tests Python 3.15 successfully.

索引モデルが issue の本文から書いたものです。