Here is an updated patch, taking Jim's and Antoine's comments into account.

Jim, I´d like to comment that I think the reason __del__ objects are uncollectable is more subtle than there being no defined order of calling the __del__ functions.  More significantly, no python code may be executed during an implicit garbage collection.
Now, it is possible that one could clean up cycles containing only one __del__ method during _expcicit_ collections (calling gc.collect()) but it hardly seems worth the effort.