Enable `flake8-bugbear` + adapt existing codebase (#967)
* enable flake8 bugbear + adapt existing codebase
* fix return if no exception
* FIX failing tests
* Update tests/testing_utils.py
Co-authored-by: Lysandre Debut <hi@lysand.re>
* fix docstring
* fastai repo prefix in fastai tests
* flake8
Co-authored-by: Lysandre Debut <hi@lysand.re>