Fix IO_ENCODER

[YuzukiTsuru]

  File "~/gitbook2pdf/gitbook2pdf/gitbook2pdf.py", line 105, in parser
    return html.unescape(ET.tostring(context).decode())
  File "src/lxml/etree.pyx", line 3437, in lxml.etree.tostring
  File "src/lxml/serializer.pxi", line 139, in lxml.etree._tostring
  File "src/lxml/serializer.pxi", line 199, in lxml.etree._raiseSerialisationError
lxml.etree.SerialisationError: IO_ENCODER