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.
We said 'bye' to Python 2 already. So let us use Python 3 for all
Unversioned Python binary should be avoided as it may be Python2 or
Python3 (depends on distribution).
Implements: blueprint drop-py2-support