Pin the versions of pep8/pyflakes here.

This way we don't need expansive blocks copied around everywhere.
hacking is developed against specific versions of libraries - use them.

Change-Id: Ie5b0e30f8067d509d19c082d5a3ebf27d737d2bd
Reviewed-on: https://review.openstack.org/30556
Reviewed-by: Joe Gordon <joe.gordon0@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Sean Dague <sean@dague.net>
Reviewed-by: Sean Dague <sean@dague.net>
Tested-by: Jenkins