Drop Python 3 from package metadata
    
    freeIPA 4.5 requires Python 2.7 and doesn't work correctly with Python
    3. Drop Python 3 from classifiers and add python_requires for ~= 2.7.5.
    
    https://pagure.io/freeipa/issue/7294
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>