Comment 1 for bug 641481

This problem occurs in 3.9 as well and probably in versions back to
2002. It seems to arise from Python's 2.7 (somewhat understandable)
intolerance of ZODB's internal weakref implementation.

I believe this will be fixed by the planned reimplementation of
persistent caches using (subclasses of) standard Python weak refs.

In the mean time, we'll probably declare Python 2.7 to be unsupported.