Make tests for Annotated work with Python 3.9 (#731)

Python 3.9+ has its own version of Annotated. Make the tests take it
into account similarly to how this is done for other backported
typing constructs.

typing_extensions/src_py3/test_typing_extensions.py

@@ -1588,13 +1588,17 @@ def test_keys_inheritance(self):
 class AnnotatedTests(BaseTestCase):
 
     def test_repr(self):
+        if hasattr(typing, 'Annotated'):
+            mod_name = 'typing'
+        else:
+            mod_name = "typing_extensions"
         self.assertEqual(
             repr(Annotated[int, 4, 5]),
-            "typing_extensions.Annotated[int, 4, 5]"
+            mod_name + ".Annotated[int, 4, 5]"
         )
         self.assertEqual(
             repr(Annotated[List[int], 4, 5]),
-            "typing_extensions.Annotated[typing.List[int], 4, 5]"
+            mod_name + ".Annotated[typing.List[int], 4, 5]"
         )
 
     def test_flatten(self):