Add support for Python 3.12

[dhermes]

Also remove support for Python 3.9

[dhermes]

Likely relevant here: https://gregoryszorc.com/blog/2023/10/30/my-user-experience-porting-off-setup.py/

(Having issues with sys.path in setup.py; it differs from within nox and outside of nox)

It seems like pip install --no-build-isolation . is the trick here. (Though I think we should start listing numpy as a build dependency.)

[epicwhale]

Any luck with supporting python 3.12?