Drive the robot backward by running both motors backward.

        :param float speed:
            Speed at which to drive the motors, as a value between 0 (stopped)
            and 1 (full speed). The default is 1.

def backward(self, speed=1):
        """
        Drive the robot backward by running both motors backward.

        :param float speed:
            Speed at which to drive the motors, as a value between 0 (stopped)
            and 1 (full speed). The default is 1.
        """
        self.left_motor.backward(speed)
        self.right_motor.backward(speed)