When the `notifies()` generator is used, notifies callbacks are not triggered

[dvarrazzo]

During notifies(), the notify handlers don't work. This is an easy to fix issue:

--- a/psycopg/psycopg/generators.py
+++ b/psycopg/psycopg/generators.py
@@ -297,6 +297,8 @@ def notifies(pgconn: PGconn) -> PQGen[list[pq.PGnotify]]:
     while True:
         n = pgconn.notifies()
         if n:
+            if pgconn.notify_handler:
+                pgconn.notify_handler(n)
             ns.append(n)
         else:
             break

Originally posted by @dvarrazzo in #962 (comment)