split.py example fails with InvalidStateError

[iangoldby]

1. Create a new Python 3.9 environment.
2. pip install aioreactive
3. Download and run split.py from examples/streams
   Expected result: see values being printed out.
   Actual result:

Exception in callback MailboxProcessor.__process_events()
handle: <Handle MailboxProcessor.__process_events()>
Traceback (most recent call last):
  File "C:\Users\Ian\.conda\envs\aioreactive\lib\asyncio\events.py", line 80, in _run
    self._context.run(self._callback, *self._args)
  File "C:\Users\Ian\.conda\envs\aioreactive\lib\site-packages\expression\core\mailbox.py", line 152, in __process_events
    cont(msg)
  File "C:\Users\Ian\.conda\envs\aioreactive\lib\site-packages\expression\core\aiotools.py", line 45, in done
    future.set_result(value)
asyncio.exceptions.InvalidStateError: invalid state
Exception in callback MailboxProcessor.__process_events()
handle: <Handle MailboxProcessor.__process_events()>
Traceback (most recent call last):
  File "C:\Users\Ian\.conda\envs\aioreactive\lib\asyncio\events.py", line 80, in _run
    self._context.run(self._callback, *self._args)
  File "C:\Users\Ian\.conda\envs\aioreactive\lib\site-packages\expression\core\mailbox.py", line 152, in __process_events
    cont(msg)
  File "C:\Users\Ian\.conda\envs\aioreactive\lib\site-packages\expression\core\aiotools.py", line 45, in done
    future.set_result(value)
asyncio.exceptions.InvalidStateError: invalid state

[davidia]

The asyncio event loop is closing immediately and cancelling the futures that do the work resulting in this error. If you stick

await asyncio.sleep(5)

at the end of the main function the error goes away.