Add support for Python 3.15

[kytta]

Description

Python 3.15.0 rc1 is out. "release candidate" means that there will be no more ABI changes, and the API over all is now stable. It is now pretty much safe to test against and to specify support for it.

This PR adds Python 3.15 support to Markdown by enabling testing on it and adding a PyPI classifier.

AI Assistance Disclosure

• No AI tools were used in preparing this PR.
• If AI tools were used, I have disclosed which ones, and fully reviewed and verified their output.

Checklist

• This PR follows the contribution guidelines.
• The code follows the Code Style Guide.
• The commit message follows the Commit Message Style Guide.
• I have added or updated relevant docs, including release notes if applicable which follow the [Documentation Style Guide](Documentation Style Guide).
• I have added or updated relevant tests. — not applicable
• I have not requested, and will not request, an automated AI review for this PR.

[waylan]

As documented here, 3.10 is scheduled for end-of-life on the same day that 3.15 is scheduled to be officially released. Therefore, we generally drop support for the outgoing Python version at the same time that we add support for the new version. Let's do that here.

[kytta]

Whoops, I missed that the two events happen at the same time here :D Sure, I will drop 3.10 when I'm back at my computer