use assert[Not]IsInstance where appropriate
diff --git a/Lib/test/test_ast.py b/Lib/test/test_ast.py
index e592032..2935a0c 100644
--- a/Lib/test/test_ast.py
+++ b/Lib/test/test_ast.py
@@ -154,7 +154,7 @@
         slc = ast.parse("x[::]").body[0].value.slice
         self.assertIsNone(slc.upper)
         self.assertIsNone(slc.lower)
-        self.assertTrue(isinstance(slc.step, ast.Name))
+        self.assertIsInstance(slc.step, ast.Name)
         self.assertEqual(slc.step.id, "None")
 
     def test_from_import(self):