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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.