post-receive-fedora-messaging: Don't attempt to decode unicode strings
    
    On Python 3, we can only decode byte sequences.
    
    subprocess.Popen(universal_newlines=True) always returns unicode stdout/sderr.
    
    Fixes https://pagure.io/fedora-infrastructure/issue/9366
    
    Signed-off-by: Miro Hrončok <miro@hroncok.cz>