Add Python 3.12 to test versions; drop Python 3.7

[mdickinson]

This PR updates the Python test versions to include Python 3.12 and exclude Python 3.7 (which is now beyond end-of-life).

[corranwebster]

Looks good.

Do we want to add python_requires to the setup.py as well?

[mdickinson]

Do we want to add python_requires to the setup.py as well?

Yes, that would be good. Done in b2fedab

[mdickinson]

Also applied some formatting fixes to setup.py in 9d562d1, since I was looking at a wall of red underlines for the setup call in VS Code.

[mdickinson]

Okay, this is messier than I was expecting. I'm going to revert back to the original commit and just merge that, then look at what it takes to remove Python 3.6 support properly.