app.py: Work around incompatibility between Tornado 6 and asyncio proactor event loop in python 3.8 on Windows

[alexweav]

In py38, asyncio made the WindowsProactorEventLoopPolicy the default setting for event loops on Windows. However, this is incompatible with Tornado. The result is that the Hub does not start on Windows on Python 3.8, with a NotImplementedError. See tornadoweb/tornado#2608 for additional context.

The workaround suggested by Tornado is to use the older WindowsSelectorEventLoopPolicy, which is the default in py37 and before. This is done via:

asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())

This workaround has already been applied in the single-user server: jupyter/notebook#5047. The content of this PR is almost identical to the single-user one, save for some formatting differences. We only alter the default setting if we're running in an environment where this issue might occur (Windows, py38, and a sufficiently advanced asyncio API).

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[alexweav]

Hello @minrk, sorry to bother, but is there a chance you or someone else might be able to review this PR? (I pinged you because the content of this PR was originally authored by you, but in the single-user server instead)

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[minrk]

Thanks for the PR! Just getting back from vacation today.