code: def _release(self):
          
          if self._in_use is None:
              # The holder is not checked out.
              return
  
          if not self._in_use.done():
              self._in_use.set_result(None)
          self._in_use = None
  
          # Deinitialize the connection proxy.  All subsequent
          # operations on it will fail.
          if self._proxy is not None:
              self._proxy._detach()
              self._proxy = None
  
          # Put ourselves back to the pool queue.
          self._pool._queue.put_nowait(self)
docs: Release this connection holder.