Task exception with Python 3.9

[Cory Riddell]

When I run my VPython script with Python 3.6, it works fine but when I switch to Python 3.9 I get a stream of errors as I manipulate the view in the browser:

Task exception was never retrieved
future: <Task finished name='Task-131' coro=<WSserver.onMessage() done, defined at C:\Users\cr\Python\3dVis2\venv\lib\site-packages\vpython\no_notebook.py:156> exception=RuntimeError('cannot schedule new futures after shutdown')>
Traceback (most recent call last):
  File "C:\Users\cr\Python\3dVis2\venv\lib\site-packages\vpython\no_notebook.py", line 189, in onMessage
    await loop.run_in_executor(None, GW.handle_msg, msg)
  File "C:\Users\cr\AppData\Local\Programs\Python\Python39-32\lib\asyncio\base_events.py", line 814, in run_in_executor
    executor.submit(func, *args), loop=self)
  File "C:\Users\cr\AppData\Local\Programs\Python\Python39-32\lib\concurrent\futures\thread.py", line 161, in submit
    raise RuntimeError('cannot schedule new futures after shutdown')
RuntimeError: cannot schedule new futures after shutdown

These start right after the initial draw. For now I'm going to stick with Python 3.6.