Import turtle lazily so tests run without a Tk library

CI on the runner failed at collection: libtk8.6 is missing, so importing
us_states_game.main imported turtle and tkinter. turtle is now imported when
TurtleScreen is created.

Task: MIL-003#7

src/us_states_game/turtle_screen.py

@@ -1,11 +1,13 @@
 """Turtle implementation of the game screen.
 
 @file turtle_screen.py
-@brief The only module that imports turtle; draws the map and asks questions.
+@brief The only module that uses turtle; draws the map and asks questions.
+
+turtle is imported when `TurtleScreen` is created, not when this module is
+imported, so the rest of the program and its tests need no Tk library.
 """
 
 import logging
-import turtle
 
 from us_states_game.constants import (
     LOGGER_NAME,
@@ -34,7 +36,12 @@ class TurtleScreen:
     """Game screen drawn with Turtle graphics; implements `GameScreen`."""
 
     def __init__(self) -> None:
-        """Open the window with the blank map."""
+        """Open the window with the blank map.
+
+        turtle (and so Tk) is imported here, at runtime.
+        """
+        import turtle  # noqa: PLC0415  # deferred: Tk may be missing at import time
+
         self.screen = turtle.Screen()
         self.screen.setup(SCREEN_WIDTH, SCREEN_HEIGHT)
         self.screen.title(SCREEN_TITLE)

tests/test_main.py

@@ -89,3 +89,16 @@ def test_game_and_states_modules_do_not_import_turtle() -> None:
     )
     result = subprocess.run([sys.executable, "-c", code], check=False)
     assert result.returncode == 0
+
+
+def test_main_imports_without_tk_library() -> None:
+    src = Path(__file__).resolve().parent.parent / "src"
+    code = (
+        "import sys; "
+        "sys.modules['tkinter'] = None; sys.modules['_tkinter'] = None; "
+        f"sys.path.insert(0, {str(src)!r}); "
+        "import us_states_game.main, us_states_game.turtle_screen; "
+        "sys.exit(1 if 'turtle' in sys.modules else 0)"
+    )
+    result = subprocess.run([sys.executable, "-I", "-c", code], check=False)
+    assert result.returncode == 0