Async notifications in psycopg3

Hello,

I have introduced async notifications handling for psycopg3. At the
moment it comes in the form of a generator (eventually async - in
asyncio sense) so that they can be used as simply as:

    for notify in conn.notifies():
        # ... process

but notifies may be also received during the normal query processing,
for which a possible way to receive them can is a callback.

I am not entirely convinced about the design so I'd be happy to
receive some feedback by anyone who has ideas about how they would use
the features. There is a more complete description and references in
<https://github.com/psycopg/psycopg3/issues/5>: happy to have a
conversation either here or on the github ticket.

Cheers,

-- Daniele