[issue12345] Add math.tau

[Tim Peters]

Tim Peters added the comment:

Hmm.  I'd test that tau is exactly equal to 2*pi.  All Python platforms  (past, present, and plausible future ones) have binary C doubles, so the only difference between pi and 2*pi _should_ be in the exponent (multiplication by 2 is exact).  Else we screwed up one or both mantissas (typed the constant wrong, and/or the C compiler is rounding incorrectly) - which would be bad.

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nosy: +tim.peters

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