Add the Pen protocol, window helpers and a recording fake turtle

The drawing functions take a Pen, the part of turtle.Turtle they use, so a test can
pass a fake that records every call. window.py is the only module that imports
turtle, and it does so inside the functions so that importing the package needs no Tk.

Task: MIL-002#1
Task: MIL-002#2

tests/test_window.py

@@ -0,0 +1,33 @@
+import sys
+
+import pytest
+
+from turtle_challenges.window import (
+    TURTLE_UNAVAILABLE_MESSAGE,
+    TurtleUnavailableError,
+    create_pen,
+    create_window,
+)
+
+
+@pytest.fixture
+def without_turtle(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Make `import turtle` fail, as on a Python without Tk."""
+    monkeypatch.setitem(sys.modules, "turtle", None)
+
+
+def test_create_window_reports_a_missing_turtle(without_turtle: None) -> None:
+    with pytest.raises(TurtleUnavailableError) as error_info:
+        create_window()
+
+    assert str(error_info.value) == TURTLE_UNAVAILABLE_MESSAGE
+    assert isinstance(error_info.value.__cause__, ImportError)
+
+
+def test_create_pen_reports_a_missing_turtle(without_turtle: None) -> None:
+    with pytest.raises(TurtleUnavailableError):
+        create_pen()
+
+
+def test_the_missing_turtle_message_names_the_debian_package() -> None:
+    assert "python3-tk" in TURTLE_UNAVAILABLE_MESSAGE