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/spirograph.py

@@ -0,0 +1,47 @@
+"""!
+@file spirograph.py
+@brief Turtle Challenge 5: draw a spirograph.
+"""
+
+import random
+
+from turtle_challenges.colors import random_color
+from turtle_challenges.constants import (
+    FASTEST_SPEED,
+    FULL_TURN_DEGREES,
+    SPIROGRAPH_GAP_DEGREES,
+    SPIROGRAPH_RADIUS,
+)
+from turtle_challenges.pen import Pen
+
+
+def draw_spirograph(
+    pen: Pen,
+    size_of_gap: float = SPIROGRAPH_GAP_DEGREES,
+    radius: float = SPIROGRAPH_RADIUS,
+    rng: random.Random | None = None,
+) -> None:
+    """!
+    @brief Draw overlapping circles, turning a little after each one.
+
+    After every circle the heading grows by @p size_of_gap, so the circles
+    overlap into a spirograph once the heading has made a full turn. The number
+    of circles is 360 divided by the gap, rounded down: `range` takes only an
+    integer, and a float such as 72.0 would raise a `TypeError`.
+
+    @param pen The turtle that draws.
+    @param size_of_gap Degrees between two circles; must be positive.
+    @param radius Radius of every circle, in turtle units.
+    @param rng Random generator for the colors; a shared one is used if
+        omitted.
+    @exception ValueError If @p size_of_gap or @p radius is not positive.
+    """
+    if size_of_gap <= 0:
+        raise ValueError(f"size_of_gap must be positive, got {size_of_gap}")
+    if radius <= 0:
+        raise ValueError(f"radius must be positive, got {radius}")
+    pen.speed(FASTEST_SPEED)
+    for _ in range(int(FULL_TURN_DEGREES / size_of_gap)):
+        pen.color(random_color(rng))
+        pen.circle(radius)
+        pen.setheading(pen.heading() + size_of_gap)