[issue34298] Avoid inefficient way to find start point in deque.index

[Raymond Hettinger]

New submission from Raymond Hettinger <raymond.hettinger at gmail.com>:

Please run some timings to show whether the improvement is significant.  Also, please sign a CLA.

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components: +Library (Lib) -Extension Modules
nosy: +rhettinger
versions:  -Python 2.7, Python 3.4, Python 3.5, Python 3.6, Python 3.7

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34298>
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