Decode the output from the shell commands if they are not already unicode
    
    We're relying on six.binary_type to distinguish between str in py2 and bytes
    in py3, both of which needed to be converted to unicode.
    
    Fixes https://pagure.io/pagure/issue/4188
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>