bpo-35252: Remove FIXME from test_functools (GH-10551)

diff --git a/Lib/test/test_functools.py b/Lib/test/test_functools.py
index 85c65d1..b89d779 100644
--- a/Lib/test/test_functools.py
+++ b/Lib/test/test_functools.py
@@ -2355,9 +2355,6 @@
         ))
         self.assertTrue(str(exc.exception).endswith(msg_suffix))
 
-        # FIXME: The following will only work after PEP 560 is implemented.
-        return
-
         with self.assertRaises(TypeError) as exc:
             @i.register
             def _(arg: typing.Iterable[str]):
@@ -2366,10 +2363,12 @@
                 # types from `typing`. Instead, annotate with regular types
                 # or ABCs.
                 return "I annotated with a generic collection"
-        self.assertTrue(str(exc.exception).startswith(msg_prefix +
-            "<function TestSingleDispatch.test_invalid_registrations.<locals>._"
+        self.assertTrue(str(exc.exception).startswith(
+            "Invalid annotation for 'arg'."
         ))
-        self.assertTrue(str(exc.exception).endswith(msg_suffix))
+        self.assertTrue(str(exc.exception).endswith(
+            'typing.Iterable[str] is not a class.'
+        ))
 
     def test_invalid_positional_argument(self):
         @functools.singledispatch