Repair the type annotations in the TokenViewBase class.

[triplepoint]

When extending the TokenViewBase class in order to override the serializer_class property, the type of the property is already established as NoneType, causing type check errors.

This PR explicitly annotates serializer_class to be Optional[Type[Serializer]], allowing for the default None and also any other class that is a child of Serializer.

Note also that the type of the return of get_serializer_class() was erroneously Serializer (an instance of Serializer), when it should be Type[Serializer] (a class who's type includes Serializer).

EDIT: Turned out BaseSerializer was the correct type, not Serializer.

[vgrozdanic]

Nice, thank you