Fix and clean up TypeVars of Serializer classes

[intgr]

I have made thunks!

• ModelSerializer was incorrectly deriving from Serializer[Any] and BaseSerializer[T]; replaced with Serializer[T].

• Added TypeVar to HyperlinkedModelSerializer class.

• Removed unnecessary Generic[] where implicit TypeVars are sufficient.

  https://mypy.readthedocs.io/en/stable/generics.html#defining-subclasses-of-generic-classes

  If there are no Generic[...] in bases, then all type variables are collected in the lexicographic order (i.e. by first appearance).