code: def check_down_connections(self):
          
          now = time.time()
  
          for db_num, marked_down_at in self._down_connections.items():
              if marked_down_at + self.retry_timeout <= now:
                  self.mark_connection_up(db_num)
docs: Iterates through all connections which were previously listed as unavailable
          and marks any that have expired their retry_timeout as being up.