use assert[Not]In where appropriate
diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index 0d43d42..8737c3d 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -350,7 +350,7 @@
                 t.start()
                 t.join()
                 l = enum()
-                self.assertFalse(t in l,
+                self.assertNotIn(t, l,
                     "#1703448 triggered after %d trials: %s" % (i, l))
         finally:
             sys.setswitchinterval(old_interval)