Comment 8 for bug 1661510

We are working on it, it seems that Kombu in-memory transport does not support "on_return" function see the issue [1].

I proposed this PR [2], it does not implment "on_return" but at least it does not crash.

1- https://github.com/celery/kombu/issues/1050
2- https://github.com/celery/kombu/pull/1053