Fix type hints of assert functions

[mschoettle]

Updated type hints in assert functions to use Iterable instead of Collection. And have assertRaisesMessage return something to use as a context manager.

This is in line with django-stubs: https://github.com/typeddjango/django-stubs/blob/master/django-stubs/test/testcases.pyi

Version 4.12.0 raises the following errors with mypy:

error: "assertRaisesMessage" does not return a value (it only ever returns None)  [func-returns-value]

error: Argument 2 to "assertQuerySetEqual" has incompatible type "QuerySet[Foo, Foo]"; expected "Collection[Any]"  [arg-type]
note: "QuerySet" is missing following "Collection" protocol member:
note:     __contains__

[kingbuzzman]

LGTM, looks like you got it from here 😉 ah, right you added it in your description, eek. As you can see, I read code first then descriptions... foot, meet mouth. 😇

[kingbuzzman]

Seems low risk. Merging.