AtlasPyFwdBwdPorts: fix pytest related build problems (ATLINFR-4022), 21.0-mac16d branch (2021.05.07.)
Remove the pytest requirement from mccabe and flake8 which all of a sudden started to create problems. Since we only build these packages and never run their tests this dependency is not really needed. It was also removed upstream, see e.g. https://github.com/PyCQA/mccabe/pull/75.
Note that this is a manual cherry-pick from !41744 (merged), for the benefit of ATLINFR-4024.