Add test cases

Author: Colin Leroy-Mira

digital_image_processing/dithering/burkes.py

@@ -40,6 +40,10 @@ def get_greyscale(cls, blue: int, green: int, red: int) -> float:
         """
         >>> Burkes.get_greyscale(3, 4, 5)
         4.185
+        >>> Burkes.get_greyscale(0, 0, 0)
+        0.0
+        >>> Burkes.get_greyscale(255, 255, 255)
+        255.0
         """
         return 0.114 * blue + 0.587 * green + 0.299 * red