Test the Proactor event loop on Windows + Python 3.8

[Quentin Pradet]

There are two issues here:

1. Tornado does not intend to support the Proactor event loop, so we need to configure Tornado to use the selector loop without changing the global policy
2. AnyIO does not support the Proactor loop yet, but we rely on AnyIO to support asyncio

So this is blocked until AnyIO supports the proactor event loop or until we add an asyncio backend.

[Thomas Grainger]

@pquentin afaik anyio 3 supports the Proactor loop now