docs: async def spawn_slaves(self, slave_addrs, slave_env_cls, slave_mgr_cls,
                             slave_kwargs=None):
          """Spawn slave environments.
  
          :param slave_addrs:
              List of (HOST, PORT) addresses for the slave-environments.
  
          :param slave_env_cls: Class for the slave environments.
  
          :param slave_kwargs:
              If not None, must be a list of the same size as *addrs*. Each item
              in the list containing parameter values for one slave environment.
  
          :param slave_mgr_cls:
              Class of the slave environment managers.
          """
          pool, r = spawn_containers(slave_addrs, env_cls=slave_env_cls,
                                     env_params=slave_kwargs,
                                     mgr_cls=slave_mgr_cls)
          self._pool = pool
          self._r = r
          self._manager_addrs = ["{}{}".format(_get_base_url(a), 0) for
                                 a in slave_addrs]