Fix zonemgr encoding issue
    
    The zonemgr validator and handler performs additional encodings for IDNA
    support. In Python 3, the extra steps are no longer necessary because
    arguments are already proper text and stderr can handle text correctly.
    
    This also fixes 'b' prefix in error messages like:
    
        option zonemgr: b'empty DNS label'
    
    Fixes: https://pagure.io/freeipa/issue/7711
    Signed-off-by: Christian Heimes <cheimes@redhat.com>