test: verify correct typing of mixins

[bradenmacdonald]

This PR makes some simplifications to PublishableEntityMixin and PublishableEntityVersionMixin so that their subclasses like ComponentVersion automatically get the correct type on their objects class property, and don't have to override it to satisfy mypy. This will slightly simplify the code as we get more publishable entities in the near future (e.g. components and units).

I also got rid of the custom PublishableEntityMixinManager and PublishableEntityVersionMixinManager classes, replacing them with the reusable WithRelationsManager that does the same thing and which now supports generic typing properly.

New type-only tests are added to assert that the typing of the manager (objects) and its returned instances are correct.

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[bradenmacdonald]

Unfortunately this line still requires specifying the type explicitly and also needs ignore[assignment] because mypy for some reason thinks that WithRelationsManager[Component] and WithRelationsManager[Self] are different things in this case. Seems like a mypy bug 🤷🏻‍♂️.

In all other cases, where we're just using WithRelationsManager directly or just relying on a mixin to provide objects, no type annotations or suppression are needed. It's just here where the override in Component seems to conflict with the override in PublishableEntityMixin (though it shouldn't).

[bradenmacdonald]

It's actually possible to remove this ClassVar[WithRelationsManager[Self]] annotation, and everything will still be correct. Though it seems VS Code / Pyright then interprets it as Manager[Self] rather than WithRelationsManager[Self], whereas mypy is unaffected. I don't have a strong opinion but it seems fine to be more explicit.

[mphilbrick211]

Friendly ping on this, @ormsbee :)

[bradenmacdonald]

This was mostly included in #278 . Now the only new thing here are some tests to verify that the typing is correct.