code: def unpack_bytes(self):
          
          num_bytes_consumed, b = decode_bytes(self.__f)
          self.__num_bytes_consumed += num_bytes_consumed
          return num_bytes_consumed, b
docs: Unpack a utf-8 string encoded as described in MQTT Version
          3.1.1 section 1.5.3 line 177.  This is a 16-bit unsigned length
          followed by a utf-8 encoded string.
  
          Returns
          -------
          int
              Number of bytes consumed
          bytes
              A bytes object extracted from the underlying stream.