Update supported Python versions: add 3.14 and drop 3.9

[pygarap]

I would like to suggest updating the officially supported Python versions for selectolax.

Python 3.9 has already reached its end of life. The official Python Developer’s Guide lists the support window and end of life dates here:
https://devguide.python.org/versions/

Given this, it probably makes sense to stop testing and advertising support for Python 3.9, and to focus on newer versions that still receive security updates.

At the same time, Python 3.14 already works with selectolax in practice, but it is not covered by the current CI jobs. It would be great to run the test suite on 3.14 as well, so that the advertised support matches what is tested.

This would keep selectolax aligned with the current Python support policy and reduce the maintenance cost of supporting an end of life interpreter.

[rushter]

I would postpone deprecating Python 3.9, because it's still heavily used. It does not require any extra effort for us to maintain it.