chore: add support for Python 3.14

[sk-]

Summary

Similar to PR #3460, we add support for Python 3.14 to the classifiers and the CI test matrix.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.