Test packages on Python 3.12 (released Oct 2, 2023)

[jpmckinney]

And update classifiers.

https://docs.python.org/3.12/whatsnew/3.12.html

• Fewer limitations on f-strings (e.g. escape and quote characters)
• TypedDict for **kwargs typing
• pathlib.Path.walk()
• tempfile.NamedTemporaryFile(delete_on_close=True)

[jpmckinney]

Closing. Python 3.12 features can be adopted later when we upgrade applications (which will coincide with when we upgrade to Ubuntu 24.04 LTS).