[issue29519] weakref spewing exceptions during finalization when combined with multiprocessing

[INADA Naoki]

INADA Naoki added the comment:

[gsutil](https://github.com/GoogleCloudPlatform/gsutil) shows massive "Exception ... ignored" message from this regression.
https://twitter.com/minimum2scp/status/890021408482316289

I think we should backport this fix to Python 2.7.14.

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nosy: +inada.naoki
versions: +Python 2.7

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<http://bugs.python.org/issue29519>
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