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

[leewz]

New submission from leewz:

I don't know if this kind of thing matters, but `from string import ChainMap` works (imports from `collections). It's used internally by `string`. This was done when ChainMap was made official:
https://github.com/python/cpython/commit/2886808d3809d69a6e9b360380080140b95df0b6#diff-4db7f78c8ac9907c7ad6231730d7900c

You can see in the above commit that `configparser` had `_ChainMap` changed to `ChainMap as _ChainMap`, but this was not done in `string`.

----------
components: Library (Lib)
messages: 263824
nosy: leewz
priority: normal
severity: normal
status: open
title: `string` exposes ChainMap from `collections`
type: behavior
versions: Python 3.5, Python 3.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue26809>
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