Add missing Find-modules and dependencies for flake8
Add Find-modules for:
- pycodestyle
- pyflakes
- flake8
Add missing dependencies for pycodestyle and pyflakes to our own build of flake8. Since LCG_96b flake8 is also available in LCG. So once all relevant branches are using it, we could decide to drop our own build.
Fixes ATLINFR-3297
Edited by Frank Winklmeier