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.