Update hacking for Python3

The repo is Python 3 now, so update hacking to version 3.0 which
supports Python 3.

Fix problems found by updated hacking version.

The tempest plugin is used on older branches as well.
We really only need hacking on master anyways,
where we no longer support python 2, so here we
make the requirement specific to python 3.

Change-Id: I885da1613e9cf754302700019a1207a279b3af2a
4 files changed