[3.13] gh-139905: Provide suggestion in error message if Generic.__init_subclass__ was not called (GH-139943) (#139956)

gh-139905: Provide suggestion in error message if `Generic.__init_subclass__` was not called (GH-139943)
(cherry picked from commit 5776d0d2e0)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>

Lib/test/test_typing.py

@@ -4582,6 +4582,34 @@ class D(Generic[T]): pass
         with self.assertRaises(TypeError):
             D[()]
 
+    def test_generic_init_subclass_not_called_error(self):
+        notes = ["Note: this exception may have been caused by "
+                 r"'GenericTests.test_generic_init_subclass_not_called_error.<locals>.Base.__init_subclass__' "
+                 "(or the '__init_subclass__' method on a superclass) not calling 'super().__init_subclass__()'"]
+
+        class Base:
+            def __init_subclass__(cls) -> None:
+                # Oops, I forgot super().__init_subclass__()!
+                pass
+
+        with self.subTest():
+            class Sub(Base, Generic[T]):
+                pass
+
+            with self.assertRaises(AttributeError) as cm:
+                Sub[int]
+
+            self.assertEqual(cm.exception.__notes__, notes)
+
+        with self.subTest():
+            class Sub[U](Base):
+                pass
+
+            with self.assertRaises(AttributeError) as cm:
+                Sub[int]
+
+            self.assertEqual(cm.exception.__notes__, notes)
+
     def test_generic_subclass_checks(self):
         for typ in [list[int], List[int],
                     tuple[int, str], Tuple[int, str],