Adopt constants.py and its test

Refs #4
Task: MIL-001#3

src/snake_game/__init__.py

@@ -0,0 +1,14 @@
+## @package snake_game
+#  @brief Snake game from Udemy's 100 Days of Code (day 24).
+#
+#  @mainpage Snake Game
+#
+#  A snake of three square segments moves across a Python turtle screen by
+#  itself and is steered with the arrow keys. It eats food, grows and keeps a
+#  score. When the head passes the wall or touches the tail the snake and the
+#  score are reset and the game goes on; the high score is kept in the file
+#  data.txt between games. The game uses the standard library only.
+#
+#  The constants of the game are in constants.py.
+
+"""Snake game from Udemy's 100 Days of Code (day 24)."""