docs: def _transition_stage(self, step, total_steps, brightness=None):
          """
          Get a transition stage at a specific step.
  
          :param step: The current step.
          :param total_steps: The total number of steps.
          :param brightness: The brightness to transition to (0.0-1.0).
          :return: The stage at the specific step.
          """
          if brightness is not None:
              self._assert_is_brightness(brightness)
              brightness = self._interpolate(self.brightness, brightness,
                                             step, total_steps)
  
          return {'brightness': brightness}