Dropping support for Python 3.9

[JamesParrott]

CPython proposes to EOL 3.9 "approximately" this month.
The last PyPy release with 3.9 support 7.3.16 was in April 2024

Supporting 3.9 is no hardship at all thanks to from __future__ import annotations, 3.12 is the minimum version that I'm looking forwrd to personally. Python 3.10 does introduce the match statement, but many if/elif blocks have already been refactored using other patterns, so I see little need for it..