Get event loop deprecate

[unkcpz]

This PR is try to force the use of custom PlumpyEventLoopPolicy for all tests. But in the end I can not get rid of all warnings since some are from nest_asyncio and pytest_asyncio where the get_event_loop is still used.

The recommended way of replacing the asyncio.get_event_loop is discussed at python/cpython#100160. I made the similar change to the PlumpyEventLoopPolicy by creating the event loop explicitly in new_event_loop.

I think the major issue came from we are using nest_asyncio to make event loop reentranable, which I introduced 4 years ago. Since the nest_asyncio is not actively maintained and it is not a decent way to do python asyncio, I'll look to see how to get rid of it by using the standard async primitives.

• The next step for this PR is to finalize the event loop create and get mechanism in custom event loop policy.