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>