Add random_color and the color palette

random_color returns an immutable (red, green, blue) tuple of integers from 0 to 255
and random_palette_color picks a named color. Both take an optional random.Random
so that tests can seed them.

Task: MIL-003#1
Task: MIL-003#2

tests/test_constants.py

@@ -1,5 +1,14 @@
 from turtle_challenges import constants
 
 
+def test_color_mode_matches_the_largest_channel_value() -> None:
+    assert constants.COLOR_MODE == constants.COLOR_CHANNEL_MAX == 255
+
+
 def test_challenge_names_are_unique() -> None:
     assert len(set(constants.CHALLENGE_NAMES)) == len(constants.CHALLENGE_NAMES)
+
+
+def test_palette_has_only_non_empty_names() -> None:
+    assert constants.COLOR_PALETTE
+    assert all(name for name in constants.COLOR_PALETTE)