tox: run flake8 on pypy and py3, drop pyflakes
tox: run flake8 on pypy and py3, drop pyflakes

flake8 depends on pyflakes, there is thus no need to run solely
pyflakes, that is redundant.

Remove pyflakes/findx entirely.
Run 'flake8 --doctests' for python 2.7, 3 and pypy. That would give us
slightly more coverage.

Change-Id: I9fddd16ef234b873441c510c8d5554eb8ac30bb4

wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core