also show asyncio task stacks

which are nicely formatted on their own

there won't be many yet, but if we transition to async def,
we will get lots more useful info

jupyterhub/utils.py

@@ -375,6 +375,7 @@ def print_stacks(file=sys.stderr):
     """
     # local imports because these will not be used often,
     # no need to add them to startup
+    import asyncio
     import resource
     import traceback
     from .log import coroutine_frames
@@ -403,3 +404,12 @@ def print_stacks(file=sys.stderr):
 
         print(''.join(['\n'] + traceback.format_list(stack)), file=file)
 
+    # also show asyncio tasks, if any
+    # this will increase over time as we transition from tornado
+    # coroutines to native `async def`
+    tasks = asyncio.Task.all_tasks()
+    if tasks:
+        print("AsyncIO tasks: %i" % len(tasks))
+        for task in tasks:
+            task.print_stack(file=file)
+