tox: Remove python 2 tests, add python 3.7, 3.8 tests
    
    Python 2 is dead for a while, and there is no point in running the tests
    now. Python 3.7 and 3.8 are available and we want to test them on
    travis.
    
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>