Make config_sources a classmethod rather than a class property

[sneakers-the-rat]

python 3.13 removed the ability to use @classmethods and @property together - https://docs.python.org/3.13/library/functions.html#classmethod

This causes a bug when using ids to load configs like:

>       globs = [src.rglob("*.y*ml") for src in cls.config_sources]
E       TypeError: 'method' object is not iterable

so this is just a PR to switch it to being a classmethod.

first committing updated ci action that tests against 3.13 to confirm failure, then committing fix.

• confirmed correctly failing in python 3.13 only - https://github.com/Aharoni-Lab/mio/actions/runs/13478817474?pr=109
• confirmed after fix no longer failing - https://github.com/Aharoni-Lab/mio/actions/runs/13478866148?pr=109

While refactoring the method, i noticed that usages were not being detected correctly by pycharm. this was because of the generic TypeVar i was using incorrectly rather than the Self annotation, which is the correct annotation for classmethods that behave like constructors of subclasses, so i also fixed that.

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📚 Documentation preview 📚: https://miniscope-io--109.org.readthedocs.build/en/109/

[coveralls]

coverage: 79.599% (+0.08%) from 79.516%
when pulling bb09028 on bugfix-config-sources-property
into 975151f on main.