Provide testing support for Python 3.12 – drop support for {3.8, 3.9}

[jGaboardi]

This PR:

• adds testing for Python 3.12 (+ update 1 test that failed in Python 3.12)
• drops testing for Python {3.8, 3.9}
• provides clear "oldest" and "no-optional" CI environments
• updates workflows accordingly
• updates docs accordingly
• couple other minor odds and ends

[jGaboardi]

@martinfleis, I am not sure why dev is failing here for installing shapely. Seems like we are doing the same thing in momepy with no problem. I am probably missing something fairly obvious...

[jGaboardi]

I guess the --pre flag must be messing something around.

--pre --index-url https://pypi.anaconda.org/scientific-python-nightly-wheels/simple --extra-index-url https://pypi.org/simple

[jGaboardi]

FWIW, these changes are now passing over in my fork. But here the PR claims to be Processing Updates