use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_pep352.py b/Lib/test/test_pep352.py
index b9645d5..a2b4ec8 100644
--- a/Lib/test/test_pep352.py
+++ b/Lib/test/test_pep352.py
@@ -73,7 +73,7 @@
                     self.verify_instance_interface(exc())
                 except TypeError:
                     pass
-                self.assertTrue(exc_name in exc_set)
+                self.assertIn(exc_name, exc_set)
                 exc_set.discard(exc_name)
                 last_exc = exc
                 last_depth = depth