Add draw_spirograph with tests

Draws int(360 / size_of_gap) circles, each in a random RGB color, turning the heading
by the gap after each. A gap that is not positive raises ValueError, and the integer
count avoids the TypeError that range raises for a float.

Task: MIL-004#2
Task: MIL-004#3

src/turtle_challenges/__init__.py

@@ -3,10 +3,11 @@
 @brief Solutions to the turtle graphics challenges of day 18 of Udemy's
 100 Days of Code.
 
-The challenges so far are @ref turtle_challenges.square.draw_square,
+The five challenges are @ref turtle_challenges.square.draw_square,
 @ref turtle_challenges.dashed_line.draw_dashed_line,
-@ref turtle_challenges.shapes.draw_shape and
-@ref turtle_challenges.random_walk.random_walk. Each takes the turtle as its
+@ref turtle_challenges.shapes.draw_shape,
+@ref turtle_challenges.random_walk.random_walk and
+@ref turtle_challenges.spirograph.draw_spirograph. Each takes the turtle as its
 first argument, so it can be tested without a window.
 """
 
@@ -14,12 +15,14 @@ from turtle_challenges.colors import random_color, random_palette_color
 from turtle_challenges.dashed_line import draw_dashed_line
 from turtle_challenges.random_walk import random_walk
 from turtle_challenges.shapes import draw_shape, draw_shapes
+from turtle_challenges.spirograph import draw_spirograph
 from turtle_challenges.square import draw_square
 
 __all__ = [
     "draw_dashed_line",
     "draw_shape",
     "draw_shapes",
+    "draw_spirograph",
     "draw_square",
     "random_color",
     "random_palette_color",