Lint the integration tests with Python 3
The integration tests under 'tests/integration' are Python 3 only.
Flake8 will fail on Python 3 syntax when running under Python 2, but for
now we wouldn't like to change this, in order to keep Python 2
compatibility of the code base.
To work around the above, set up a separate tox environment to lint the
integration tests, and exclude linting the integration tests when the
rest of the code is checked.
Signed-off-by: Hunor Csomortáni <csomh@redhat.com>