Allow Python 3.9 as minimum and update CI.

[joeced]

Run CI scripts on Python versions 3.9 - 3.12 and updated pyproject.toml accordingly.

[watermarkhu]

It seems that it passes on 3.10 but fails on 3.9.

Looking at the test log, it is probably only due to the annotations with the PEP604 style. This should pass also on 3.9 after adding

from __future__ import annotations

[watermarkhu]

Hi @joeced I've added support for Python 3.9 in #44