Set the sensitivity value.

        Valid values are 31 (lowest) to 254 (highest), default is 69.

def set_sensitivity(self, sensitivity=DEFAULT_SENSITIVITY):
        """Set the sensitivity value.

        Valid values are 31 (lowest) to 254 (highest), default is 69.
        """
        if sensitivity < 31:
            self._mtreg = 31
        elif sensitivity > 254:
            self._mtreg = 254
        else:
            self._mtreg = sensitivity
        self._power_on()
        self._set_mode(0x40 | (self._mtreg >> 5))
        self._set_mode(0x60 | (self._mtreg & 0x1f))
        self._power_down()