Better runtime TypedDict (GH-17214)

This patch enables downstream projects inspecting a TypedDict subclass at runtime to tell which keys are optional.

This is essential for generating test data with Hypothesis or validating inputs with typeguard or pydantic.

Lib/test/test_typing.py

@@ -3741,6 +3741,13 @@ def test_total(self):
         self.assertEqual(Options(log_level=2), {'log_level': 2})
         self.assertEqual(Options.__total__, False)
 
+    def test_optional_keys(self):
+        class Point2Dor3D(Point2D, total=False):
+            z: int
+
+        assert Point2Dor3D.__required_keys__ == frozenset(['x', 'y'])
+        assert Point2Dor3D.__optional_keys__ == frozenset(['z'])
+
 
 class IOTests(BaseTestCase):