Make compatible with python 3.8 and 3.9

[fralau]

The problem is with type hints.

Stick to type hinting and syntax that works in 3.8:

✅ Use typing.List, typing.Dict, typing.Optional, etc.

🚫 Avoid list[str], int | None

[fralau]

Specify

[project]
requires-python = ">=3.8"