docs: def _server_whitelist(self):
          '''
          Returns list of servers that have not errored in the last five minutes.
          If all servers have errored in the last five minutes, returns list with
          one item, the server that errored least recently.
          '''
          whitelist = []
          for server in self.servers:
              if (server not in self.last_error
                      or self.last_error[server] < time.time() - self.PENALTY_BOX_TIME):
                  whitelist.append(server)
          if not whitelist:
              whitelist.append(sorted(
                  self.last_error.items(), key=lambda kv: kv[1])[0][0])
          return whitelist