docs: async def spawn(self):
          """Spawn the command wrapped in this object as a subprocess."""
          self._server._pending_set.add(self)
          await self._server._sem.acquire()
          self._subprocess = await asyncio.create_subprocess_shell(
              self._cmd,
              stdout=asyncio.subprocess.PIPE,
              stderr=asyncio.subprocess.PIPE)
          self._began_at = datetime.now()
          if self in self._server._pending_set:
              self._server._pending_set.remove(self)
          self._server._running_set.add(self)
          self._began_running_evt.set()