transformers

345c23a6 - Replace (TF)CommonTestCases for modeling with a mixin.

Replace (TF)CommonTestCases for modeling with a mixin. I suspect the wrapper classes were created in order to prevent the abstract base class (TF)CommonModelTester from being included in test discovery and running, because that would fail. I solved this by replacing the abstract base class with a mixin. Code changes are just de-indenting and automatic reformattings performed by black to use the extra line space.