Add the Player class and the Up key binding

Task: MIL-003#1
Task: MIL-003#2
Task: MIL-003#3

tests/conftest.py

@@ -0,0 +1,18 @@
+"""Shared fixtures. Tests that need a window skip when no display is available."""
+
+import tkinter
+import turtle
+
+import pytest
+
+
+@pytest.fixture(scope="session")
+def screen() -> turtle.TurtleScreen:
+    """A single hidden turtle screen shared by the tests that draw."""
+    try:
+        game_screen = turtle.Screen()
+    except tkinter.TclError:
+        pytest.skip("no display available")
+    game_screen.tracer(0)
+    game_screen.getcanvas().winfo_toplevel().withdraw()
+    return game_screen