Define a function is_dark that consumes a Pixel and returns a boolean indicating whether or not the Pixel is dark.

A Pixel is a dataclass with three float fields: red, green, and blue. These float will go from 0.0 (dark) to 1.0 (light).

To determine if a pixel is dark, calculate the average of the three fields and test if the result is less than or equal to 0.5.