Add required import ^^;

Author: encukou

Lib/test/test_concurrent_futures/util.py

@@ -10,7 +10,7 @@
 from concurrent.futures.process import _check_system_limits
 
 from test import support
-from test.support import threading_helper
+from test.support import threading_helper, warnings_helper
 
 
 def create_future(state=PENDING, exception=None, result=None):