Appropriate asyncio.set_event_loop_policy to make scripts usable on Windows

[rpahut]

Is your feature request related to a problem? Please describe.
Running nominatim-cli.py import on Windows eventually runs into an exception at psycopg.AsyncConnection.connect call:
Psycopg cannot use the 'ProactorEventLoop' to run in async mode. Please use a compatible event loop, for instance by setting 'asyncio.set_event_loop_policy(WindowsSelectorEventLoopPolicy())'

Describe the solution you'd like
The official solution to this exception is to set the event loop compatible with the OS some time before the async work:

asyncio.set_event_loop_policy(
    asyncio.WindowsSelectorEventLoopPolicy()
)

This removes the exception, and as far as I can tell there are no other fatal issues preventing the script from being used on Windows.

Describe alternatives you've considered
Simple ad-hoc modification to the nominatim-cli.py works, but set_event_loop_policy can be called almost anywhere before the connect and does not have to be a part of nominatim scripts.

[lonvia]

We could add an appropriate line but I have no way to test if that actually works. Anybody interested to set up CI testing on a Windows machine?