code: async def _get_smallest_env(self):
          
          async def slave_task(mgr_addr):
              r_manager = await self.env.connect(mgr_addr, timeout=TIMEOUT)
              ret = await r_manager.get_agents(addr=True)
              return mgr_addr, len(ret)
  
          sizes = await create_tasks(slave_task, self.addrs, flatten=False)
          return sorted(sizes, key=lambda x: x[1])[0][0]
docs: Get address of the slave environment manager with the smallest
          number of agents.