[RFE] Add --flake8-checks-only option

[jcpunk]

In my CI Environment, I test against multiple versions of python. It would be nice if I could easily fire off a pipleline that just does the flake8 tests so I can easily isolate regressions in unit tests vs flake8 complaints.

Some sort of --flake8-checks-only option or similar would be ideal for helping to isolate the source of the complaints.

[bouweandela]

You can use pytest --flake8 -m flake8