move pyflakes rule near the pep8 one

For sake of clarity.

Change-Id: I07457cb347ee75a9fe20bcb593cb4ab907f14dd1
pep8/pyflakes should trigger when setup.cfg is changed

Spotted on https://gerrit.wikimedia.org/r/#/c/112036/ which did not
change any python files and thus did not match the pep8/pyflakes files:
rules.

Change-Id: I4faa5ec037b668950b69d070e5611a87252c36ed