use assert[Not]IsInstance where appropriate
diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index 0360ea9..5daf822 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -154,8 +154,7 @@
         # Wait for the thread to finish.
         mutex.acquire()
         self.assertIn(tid, threading._active)
-        self.assertTrue(isinstance(threading._active[tid],
-                                threading._DummyThread))
+        self.assertIsInstance(threading._active[tid], threading._DummyThread)
         del threading._active[tid]
 
     # PyThreadState_SetAsyncExc() is a CPython-only gimmick, not (currently)