It was a bad idea to revert the change, because we may now forget about
the problem rather than fix it.

It is likely that the "dictionary changed size during iteration" occurs
because a GC collection gets triggered during the iteration on the
weakdict. A GC collection can destroy some objects in that case because
keeping an exception around (which the unittest change does) usually
creates reference cycles through the frame's locals.

It seems that WeakValueDictionary.items() isn't very robust in py3k ;)
We probably need to add list-returning variants, e.g. a listitems()
method (and, probably, listkeys() and listvalues() too)?