Ensure the name of the headers are always of the correct type
    
    This comes with one twist, in python3 the lines we get from
    subprocess are bytes but we don't want to convert them entirely
    to unicode, we only want to convert the header's name, and
    there using `str()` won't work, we need a full `.decode()`.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>