no longer call set_event_loop in IOLoop.start()

[graingert]

asyncio.set_event_loop was only needed to make asyncio.get_event_loop() work correctly on Python 3.5.3 and earlier, which is no longer supported by tornado

This change is motivated by asyncio.run no longer calling set_event_loop

[graingert]

because asyncio.get_event_loop() calls asyncio.get_running_loop() this set_event_loop() is now redundant

[graingert]

see also #3156 cc @minrk

[graingert]

It looks like the equivalent change in asyncio.run to avoid calling set_event_loop might be causing issues in downstream projects

[graingert]

here's the issue: python/cpython#93896

[bdarnell]

Yeah, it looks like set_event_loop does more than just determine the result of a future get_event_loop; it is (at least) necessary for the child watcher system so I think it needs to stay until that system is gone. Tornado doesn't directly use child watchers but I think Tornado applications could so we need to stay compatible with it.