Improving `Connection.notifies()` in Psycopg 3.2

Hello,

Psycopg 3 introduced a `Connection.notifies()` generator, yielding
async notifications as they are received:

for n in conn.notifies():
do_something_with(n)

However it proved harder to use than expected:

- during the loop, the connection cannot be used to run queries (and
some would be pretty useful, such as LISTEN/UNLISTEN to change
the channels to listen at);
- it is not clear how to stop the generator. If assigned to a variable
`gen`, it can be closed with `gen.close()` but it must be done from an
external task/thread if the generator is waiting and I seem to
remember that it's not possible to call close on an async generator
from an external task.

In https://github.com/psycopg/psycopg/pull/673 I propose to add
optional parameters to the function:

- `timeout`: max time to wait for notifications;
- `stop_after`: stop after receiving this number of notifications (may
return more than what requested if received in the same batch);

I think these additions should make the generator easier to use, but
I'm not completely sold on its interface.

What do you think? Comments are welcome.

Cheers

-- Daniele