[issue12345] Add math.tau

[Zachary Ware]

Zachary Ware added the comment:

Builds fine on Windows.  Happily, `math.tau == 2*math.pi` is True.

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nosy: +zach.ware
stage: needs patch -> commit review
versions: +Python 3.6

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<http://bugs.python.org/issue12345>
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