Drop support for Python 3.9?

[hoechenberger]

I would love to drop support for Python 3.9, which would allow us to write types such as

Optional[Union[str, float]]

as

None | str | float

which is much more readable. I'm mostly thinking about the configuration file here, which is immediately user-facing.

Thoughts?

[larsoner]

Sure, 3.10 came out over 2 years ago and it's even within SPEC0 to drop it:

[hoechenberger]

I don't like SPEC but I do like dropping old versions :D

I can take a stab at this next week if you want. Unless you insist on having a cozy Sunday afternoon project :)

[larsoner]

Feel free to tackle it whenever you get time, probably along with #907