Add Python 3.13 support

[dantownsend]

Python 3.13 will be out very soon, which also means Python 3.8 will be end of life.

We should update our CI, so we test Python 3.13 and not 3.8.

We should also update the classifiers in setup.py.

Now we no longer support Python 3.8 we can change the type annotations from t.List[str] to list[str] etc. This will be a massive change, so best done in another PR.