bpo-43901: Lazy-create an empty annotations dict in all unannotated user classes and modules (#25623)

Change class and module objects to lazy-create empty annotations dicts on demand.  The annotations dicts are stored in the object's `__dict__` for backwards compatibility.

Lib/test/test_module.py

@@ -286,6 +286,60 @@ a = A(destroyed)"""
             melon = Descr()
         self.assertRaises(RuntimeError, getattr, M("mymod"), "melon")
 
+    def test_lazy_create_annotations(self):
+        # module objects lazy create their __annotations__ dict on demand.
+        # the annotations dict is stored in module.__dict__.
+        # a freshly created module shouldn't have an annotations dict yet.
+        foo = ModuleType("foo")
+        for i in range(4):
+            self.assertFalse("__annotations__" in foo.__dict__)
+            d = foo.__annotations__
+            self.assertTrue("__annotations__" in foo.__dict__)
+            self.assertEqual(foo.__annotations__, d)
+            self.assertEqual(foo.__dict__['__annotations__'], d)
+            if i % 2:
+                del foo.__annotations__
+            else:
+                del foo.__dict__['__annotations__']
+
+    def test_setting_annotations(self):
+        foo = ModuleType("foo")
+        for i in range(4):
+            self.assertFalse("__annotations__" in foo.__dict__)
+            d = {'a': int}
+            foo.__annotations__ = d
+            self.assertTrue("__annotations__" in foo.__dict__)
+            self.assertEqual(foo.__annotations__, d)
+            self.assertEqual(foo.__dict__['__annotations__'], d)
+            if i % 2:
+                del foo.__annotations__
+            else:
+                del foo.__dict__['__annotations__']
+
+    def test_annotations_getset_raises(self):
+        # module has no dict, all operations fail
+        foo = ModuleType.__new__(ModuleType)
+        with self.assertRaises(TypeError):
+            print(foo.__annotations__)
+        with self.assertRaises(TypeError):
+            foo.__annotations__ = {}
+        with self.assertRaises(TypeError):
+            del foo.__annotations__
+
+        # double delete
+        foo = ModuleType("foo")
+        foo.__annotations__ = {}
+        del foo.__annotations__
+        with self.assertRaises(AttributeError):
+            del foo.__annotations__
+
+    def test_annotations_are_created_correctly(self):
+        from test import ann_module4
+        self.assertTrue("__annotations__" in ann_module4.__dict__)
+        del ann_module4.__annotations__
+        self.assertFalse("__annotations__" in ann_module4.__dict__)
+
+
     # frozen and namespace module reprs are tested in importlib.