The repo is Python 3 now, so update hacking to version 3.0 which
supports Python 3.
Fix problems found.
Update local hacking checks for new flake8.
Remove hacking and friends from lower-constraints, they are not needed
to be installed at run-time.
Change-Id: I602e3f3dd3f158799f910d73ac9c759efe15fcc0
1d9a681b83