Update to newest hacking version, it has only minimal changes and full
support for Python 3.
Python modules related to coding style checks (listed in blacklist.txt in
openstack/requirements repo) are dropped from lower-constraints.txt
as they are not actually used in tests (other than pep8).
Change-Id: Id4b5f045563640f4b9c73ecafd4ebf8d3ff2c269