fixing event manager so it can clear the queue

[skirdey]

When running telephonyserver, some calls would not terminate appropriately, incurring high cost in Twilio as they could get stuck for 250 minutes even if they lasted 10 seconds.
This was due to EventsManager not clearing the queue appropriately and stopping Call termination process from completing.

In the logs it would look like this:

  File "/home/ubuntu/venv/lib/python3.10/site-packages/vocode/streaming/streaming_conversation.py", line 687, in terminate
    await self.events_manager.flush()
  File "/home/ubuntu/venv/lib/python3.10/site-packages/vocode/streaming/utils/events_manager.py", line 36, in flush
    await self.handle_event(event)
    TypeError: object NoneType can't be used in 'await' expression

it seems like futures fixtures messing up how async definition is handled within the class