Mmmh, the problem with the list(X.items()) idiom is that it's
thread-safe only if X.items() is implemented in C. Otherwise X can be
mutated if there is a thread-switch while executing bytecode in X.items().

In weakref.py (line 103), by replacing:
    for key, wr in self.data.items():
with:
    for key, wr in list(self.data.items()):

This particular error should disappear.

But this doesn't say why the dictionary is mutated at all. Does
multiprocessing (or at least that particular test) launch several
threads in a given process? Otherwise there may be something fishy going on.