Python 3.15.0 added to actions/python-versions

Bit of a niche link, but I've been waiting for this for a couple of days - I even told ChatGPT to check for it:

Clone https://github.com/actions/python-versions and git pull once an hour until they add the stable 3.15 - then tell me about it

Now that this has landed, you can add "3.15" to a GitHub Actions testing matrix to run tests against the new Python 3.15.0 release.