fix: update asyncio event loop policy on Windows

[radluz]

Fix for Windows incompatibility

On Windows, Psycopg is not compatible with the default ProactorEventLoop. Please use a different loop, for instance the SelectorEventLoop.

For instance, you can use, early in your program:

asyncio.set_event_loop_policy(
    asyncio.WindowsSelectorEventLoopPolicy()
)

Source: https://www.psycopg.org/psycopg3/docs/advanced/async.html

[waketzheng]

Why platform.system?
See https://stackoverflow.com/questions/4553129/when-to-use-os-name-sys-platform-or-platform-system