[issue26809] `string` exposes ChainMap from `collections`

[Martin Panter]

Martin Panter added the comment:

I’m not that fussed if the _ChainMap name is backported. I just thought it is safer to not do it; similar reasoning to why I only committed Issue 23883 patches to 3.6. I certainly think backporting __all__ is reasonable.

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<http://bugs.python.org/issue26809>
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