Instead of different stages for pyflakes and pep8 use pylint plugins

[rinslow]

pytest-flakes https://pypi.org/project/pytest-flakes/
pytest-pep8 https://pypi.org/project/pytest-pep8/
pytest-pylint https://pypi.org/project/pytest-pylint
pytest-sphinx pytest-sphinx-0.2.1

[osherdp]

Well, that's a great idea!
I really liked the caching... Too bad it's not implemented in the pylint plugin