psycopg3 support

[gavinwahl]

  File "dpq/commands.py", line 82, in handle
    self.wait()
  File "dpq/commands.py", line 89, in wait
    count = len(self.queue.wait(self.delay))
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "dpq/queue.py", line 60, in wait
    connection.connection.poll()
    ^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Connection' object has no attribute 'poll'

https://www.psycopg.org/docs/advanced.html#async-notify
https://www.psycopg.org/psycopg3/docs/advanced/async.html#asynchronous-notifications