As mentioned in issue7060, weak dict iterators are easily broken by
cyclic garbage collection changing the size of the underlying dict storage:

  File "/home/rdmurray/python/py3k/Lib/weakref.py", line 121, in items
    for wr in self.data.values():
RuntimeError: dictionary changed size during iteration

One possible solution is to delay all removals until all iterators are
done. Here is a context manager-based solution.

> delay all removals until all iterators are done

+1

This new patch makes it possible to mutate the dict without messing with
the delayed removal when an iterator exists.

It occurs weaksets have the same problem, here is a new patch fixing
them as well.