use assert[Not]In where appropriate
diff --git a/Lib/test/test_copy.py b/Lib/test/test_copy.py
index 823e9b3..ccecfbd 100644
--- a/Lib/test/test_copy.py
+++ b/Lib/test/test_copy.py
@@ -622,7 +622,7 @@
         x, y = C(), C()
         # The underlying containers are decoupled
         v[x] = y
-        self.assertFalse(x in u)
+        self.assertNotIn(x, u)
 
     def test_copy_weakkeydict(self):
         self._check_copy_weakdict(weakref.WeakKeyDictionary)