AtlasPyFwdBwdPorts: fix pytest related build problems (ATLINFR-4022)

Frank Winklmeier requested to merge fwinkl/athena:atlaspy_pytest_21.2 into 21.2

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.

Manual sweep of !41744 (merged).

Merge request reports