Drop support for Python 3.9, 3.8

[Jongy]

With #790 we are fully based on 3.10 now.
This means we can drop support (linting, testing) of old versions, and start using new Python features (e.g I'm tired of error: "dict" is not subscriptable, use "typing.Dict" instead).

I'll let the new version post #790 run for a while, and if all is well after a month or so, we'll remove support from the old versions.