Update annotations to Python 3.10, drop 3.8 and 3.9 support

[robinmackaij]

With Python 3.8 end of life, it's now possible to update the minimum version to 3.9 and update the annotations using the typing features for 3.9.

However, the main "cleanup" in annotations becomes possible with Python 3.10, allowing to drop the use of Union and Optional by replacement with | None.

While bumping to 3.10 at this time means dropping 3.9 support early, I expect the impact to be limited considering the adaptation of Python 3.9 has not been significant (usage statistics show the Python 3.9 share to be less than that of 3.8 and 3.10 as of december 2024).