py3: run already ported scripts under py3 by default
    
    To prevent regressions in py3, all ported scripts should be run by py3
    by default.
    
    This is temporary and will be removed once porting to py3 is done
    
    https://pagure.io/freeipa/issue/4985
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>