[python-tulip] Windows support: no proactor event loop policy

[Victor Stinner]

Hi,

While documenting limitations on Windows and ran some tests on
Windows, I saw that there is no event loop policy for the
ProactorEventLoop. Is it a choice or we just forgot it?

By the way, Guido asked me if get_event_loop() is supposed to return
None or raise an error if there is no event loop in the current
thread.

Victor