Turn on/off the callback keepalive.  If ``interval`` seconds pass with
        no data read from or written to the socket, the callback will be
        executed and the timer will be reset.

def set_keepalive(self, interval, callback):
        """
        Turn on/off the callback keepalive.  If ``interval`` seconds pass with
        no data read from or written to the socket, the callback will be
        executed and the timer will be reset.
        """
        self.__keepalive_interval = interval
        self.__keepalive_callback = callback
        self.__keepalive_last = time.time()