Inherit from faked classes in tests to satisfy mypy

[piotrszyma]

After adding typing of APIClient.get method to drf stubs project typeddjango/djangorestframework-stubs#330 mypy started complaining about these classes in tests:

drf_source/tests/test_reverse.py:43: error: Incompatible types in assignment (expression has type "MockVersioningScheme", variable has type "Optional[BaseVersioning]")
drf_source/tests/test_reverse.py:50: error: Incompatible types in assignment (expression has type "MockVersioningScheme", variable has type "Optional[BaseVersioning]")
drf_source/tests/test_versioning.py:136: error: Incompatible types in assignment (expression has type "Type[FakeResolverMatch]", variable has type "Optional[ResolverMatch]")
drf_source/tests/test_versioning.py:209: error: Incompatible types in assignment (expression has type "Type[FakeResolverMatch]", variable has type "Optional[ResolverMatch]")
drf_source/tests/test_versioning.py:260: error: Incompatible types in assignment (expression has type "Type[FakeResolverMatch]", variable has type "Optional[ResolverMatch]")

This PR fixes these errors:

• FakeResolverMatch now inherits ResolverMatch,
• MockVersioningScheme now inherits from BaseVersioning

what satisfies the type system.