Add support for Python 3.14 and drop EOL 3.8-3.9

[hugovk]

Also test PyPy3.11, because the latest releases no longer contain PyPy3.9 or PyPy3.10:

https://pypy.org/posts/2025/07/pypy-v7320-release.html

(We could also drop Python 3.9, it's EOL next month: https://devguide.python.org/versions/)

Edit: 3.9 now also dropped.