tox: Remove pytest version
    
    pytest 4.0 is too old. It was needed when we supported python 2 but we
    don't since Jan 2020. Lets use latest pytest.
    
    With newer pytest six is missing. We need to remove usage of six but
    lets starts by adding it in the tests.
    
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>