We need to hold a reference to the pool because the regression is that the pool may be destroyed while one of its iterators is still alive if no references are left to the pool. In this case, iterating over the iterator of the pool hangs. The solution links the lifetime of the pool to the lifetime of the iterator (basically: keeping the pool alive while the iterator is alive). When the iterator is exhausted, the references to the pool are cleaned.

The issue in #8450 is different, is a reference cycle between the Pool object, and the self._worker_handler Thread object, which is more fundamental.

In this case, the cycle needs to exist while the iterator is alive: all objects that depend on the pool need to make sure that the pool is alive while they are still in use (a non-consumed iterator...etc), otherwise hangs or inconsistent state can happen. This did not happen before #8450 because of the reference cycle between the Pool object and self._worker_handler prevented the pool to be destroyed.

This PR makes sure to break the cycle when the pool objects are not needed anymore.