docs: def utf8_bytes_string(s):
      """Convert a string to a bytes string (if necessary, encode in utf8)"""
      if sys.version_info[0] == 2:
          if isinstance(s, str):
              return s
          else:
              return s.encode('utf8')
      else:
          if isinstance(s, str):
              return bytes(s, encoding='utf8')
          else:
              return s