Support Python 3.12

[mtth]

Follow up to #192 (see #192 (comment)).

[musicinmybrain]

I think I am going to work around this in the Fedora package by adding zombie-imp to install_requires so that, when from importlib import load_source raises an ImportError on Python 3.12, the from imp import load_source fallback works. The zombie-imp project is plastered with warnings not to use it, so this not a good long- or even medium-term solution. I’m not sure I will have time to dig into finding the correct way to replace load_source in this project, though.

[mtth]

FYI just noticed Python 3.12's release notes have a section on replacing imp.load_source (found via python/cpython#104212 (comment)).

[musicinmybrain]

Nice, that’s both new and useful. I’m happy to try it out and offer a PR. I don’t really have a high-level understanding of the autoload system to which this belongs; do we want the module cached in sys.modules or not?

[mtth]

do we want the module cached in sys.modules or not?

Leaning towards caching since client classes are cached and executing the module multiple times might otherwise lead to race conditions.