Support Python>=3.9

[mlprt]

Feedbax is built on some features introduced in Python 3.11, such as Self.

Some users may want to use Feedbax along with some packages that do not support 3.11 yet. Ideally we would support all active Python versions, though 3.8 is reaching end-of-life this year and I think we can exclude it.

I intend to keep developing in 3.11, mainly because of the newer typing features. I don't foresee problems with 3.9 support, aside from the effect on typing. As for typing, I think support for 3.9 would involve simplifying some annotations, e.g. replacing with Any or flagging with type: ignore.