use assert[Not]IsInstance where appropriate
diff --git a/Lib/test/test_userstring.py b/Lib/test/test_userstring.py
index b99581d..b2d2e67 100755
--- a/Lib/test/test_userstring.py
+++ b/Lib/test/test_userstring.py
@@ -114,7 +114,7 @@
         s = self.type2test("foobar")
         s2 = s.immutable()
         self.assertEqual(s, s2)
-        self.assertTrue(isinstance(s2, UserString))
+        self.assertIsInstance(s2, UserString)
 
     def test_iadd(self):
         s = self.type2test("foo")