Py3: Replace six.moves imports
    
    Replace six.moves and six.StringIO/BytesIO imports with cannonical
    Python 3 packages.
    
    Note: six.moves.input behaves differently than builtin input function.
    Therefore I left six.moves.input for now.
    
    See: https://pagure.io/freeipa/issue/7715
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>