As the last update for dropping Python2 support, remove helper functions
introduced to run unittests on both of py2 and py3 with six.
Signed-off-by: Yasufumi Ogawa <email@example.com>
The repo is Python 3 now, so update hacking to version 3.0 which
supports Python 3.
Fix problems found by updated hacking version.
Update local hacking checks for new flake8.
Remove hacking and friends from lower-constraints, they are not
needed in installations.