End of life Python 3.9

[S-Linde]

Oktober 2025 marks the end of life of Python 3.9 (see).

If we stop supporting python39 at this point it would mean that annotations like int | float no longer need an from __future__ import annotations fix at the top of the file. These should then thus be removed.

Furthermore, type aliases can use TypeAlias from the typing library and should be updated accordingly.