Set send_tc_flg to send Topology Change BPDU.

def transmit_tc_bpdu(self):
        """ Set send_tc_flg to send Topology Change BPDU. """
        if not self.send_tc_flg:
            timer = datetime.timedelta(seconds=self.port_times.max_age
                                       + self.port_times.forward_delay)
            self.send_tc_timer = datetime.datetime.today() + timer
            self.send_tc_flg = True