Auto inherit from TypedModelMeta

[UnknownPlatypus]

I have made things!

(Review per commit advised, the first one is a minor cleanup)

Implement the suggestion in #1375 (comment) providing automatic type-checking of Django models Meta class by artificially inserting TypedModelMeta in the mro.

The error message might be slightly confusing since the user did not add an explicit base class but I think is comprehensible enough and that the value outweigh the slightly weird message. For ex:

Incompatible types in assignment (expression has type "int", base class "TypedModelMeta" defined the type as "bool")

I've also tested this change on a rather large Django codebase (1M python LoC) adding errors that were all caught properly.

[sobolevn]

Please, see why test is failing

[sobolevn]

Do we need to add this base class to ClassDef as well?

[UnknownPlatypus]

I don't think because we could endup with multiple TypedModelMeta in the mro for multiple inheritance cases.

[UnknownPlatypus]

Please, see why test is failing

I think I found the issue, this was because self._new_dependency("django_stubs_ext.db.models") was not called for dangling abstract django models because of how self.django_context.model_modules collect modules (It uses django logic to get concrete models, and then walk the mro to find abstract ones, hence missing dangling abstract models)

So depending on how mypy was resolving the module deps, we could sometime check a file before "django_stubs_ext.db.models" was checked, hence the flaky test. And yeah, abstract = 7 basically mean abstract = True.

I've updated the test to have concrete models at some point. Supporting dangling abstract model will add too much complexity, I don't think it's worth it

[sobolevn]

Great stuff! Thanks again!