Fix PageNumberPagination.paginate_queryset() return type

[richardxia]

It looks like the return type for PageNumberPagination.paginate_queryset() was incorrectly set to Optional[List[Page]], when it should really be an optional list of the QuerySet's underlying model type. The actual implementation in DRF shows that it returns list(self.page), which transforms the page to a list, not wrapping it with a list, since Page is itself a sequence of model objects. I have confirmed this by running actual code on djangorestframework 3.12 and Django 3.2 and confirming that I'm getting a list of model instances, not a list of pages of model instances.

The type stubs for the other two Pagination classes here look like they have the correct return type, which provide a parameter _MT for the QuerySet argument and uses that parameter in the return type, so this change really just brings PageNumberPagination up to date with the other two.

I also added a type checking test for this, although I'm not sure how useful it is, since it's pretty trivial. Let me know if you'd prefer that the test be removed.

[sobolevn]

Thanks!

[richardxia]

I believe the typechecking test that is failing doesn't have anything to do with my changes here. I diff'd the console output between my failing CI build here and the latest succeeding build in master from 23 days ago, and the only difference appears to be the following:

drf_source/tests/test_fields.py:2016: error: Argument 1 to "ListField" has incompatible type "CharField"; expected "bool"

I tried running the type checking tests on my local checkout, and even on the master branch, that test fails for me. I took a look at the django-rest-framework codebase itself, and it looks like that was a brand new test that was merged 20 days ago in encode/django-rest-framework#7632.

I pushed up a new commit to add a new exclusion for that test, but let me know if you prefer me to submit a separate PR for that, since it's technically unrelated to any of my changes here.

[sobolevn]

Great! Thanks again!