Drop Python 3.9 support

[jank324]

The drop schedule in SPEC 0 recommends that Python 3.9 support should be dropped in Q4 2023. Some packages like scipy have recently dropped support for 3.9. So we should consider dropping support at some point, at the very latest probably once it creates issues.

There is currently one item that I think means we must still support 3.9, which is the fact that PyDOOCS is currently only supported on Python 3.9.