add an additional case for idle ThreadPoolExecutor threads

this is what they look like in Python 3.7

jupyterhub/utils.py

@@ -437,6 +437,9 @@ def print_stacks(file=sys.stderr):
             if (
                 last_frame[0].endswith('threading.py')
                 and last_frame[-1] == 'waiter.acquire()'
+            ) or (
+                last_frame[0].endswith('thread.py')
+                and last_frame[-1].endswith('work_queue.get(block=True)')
             ):
                 # thread is waiting on a condition
                 # call it idle rather than showing the uninteresting stack