use assert[Not]In where appropriate
diff --git a/Lib/test/test_mimetools.py b/Lib/test/test_mimetools.py
index 9f5bf77..86a26dc 100644
--- a/Lib/test/test_mimetools.py
+++ b/Lib/test/test_mimetools.py
@@ -31,7 +31,7 @@
         s = set([""])
         for i in xrange(100):
             nb = mimetools.choose_boundary()
-            self.assertTrue(nb not in s)
+            self.assertNotIn(nb, s)
             s.add(nb)
 
     def test_message(self):