Add python_requires to Python package metadata
    
    freeIPA 4.6 and 4.7 requires Python 2.7 or >= 3.5.
    
    https://pagure.io/freeipa/issue/7294
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>