Store asyncio.Task to Avoid GC

[Brendan-Blanchard]

I was looking through the websocket code recently and noticed that a Task was being created and not stored anywhere. The async docs call this out as a problem since created Tasks can be garbage-collected if they're not referenced by anything, thus stopping them mid-execution.

I haven't seen this occur, but it's an easy fix to store it on the class, and from a quick reading I don't think it should have any side effects.

Link (second badge labeled "Important"): https://docs.python.org/3/library/asyncio-task.html#creating-tasks

[cb-heimdall]

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