Synchronize state with switch.

def update(self, retry=2) -> None:
        """Synchronize state with switch."""
        try:
            _LOGGER.debug("Updating device state.")
            key = ON_KEY if not self._flip_on_off else OFF_KEY
            self.state = self._device.readCharacteristic(HANDLE) == key
        except (bluepy.btle.BTLEException, AttributeError):
            if retry < 1 or not self._connect():
                self.available = False
                _LOGGER.error("Failed to update device state.", exc_info=True)
                return None
            return self.update(retry-1)
        self.available = True
        return None