Add some a2x decoding fixes from upstream PR#5
    
    PR: https://github.com/asciidoc/asciidoc-py3/pull/5
    
    Commits:
    6469317 Remove unnecessary decode in a2x (Matthew Peveler)
    684913e Fix decoding of file that specifies encoding in header tag in a2x (Matthew Peveler)
    8369a97 re-add --nonet option (Matthew Peveler)
    
    These changes fix failures in building cgit's documentation (and
    hopefully other similar decoding failures).