[issue12345] Add math.tau

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

In general, the volume, in n-dimensional Euclidean space, of the unit n-ball, is given by

    V_n = pi**(n/2) / gamma(n/2+1)

or

    V_n = (tau/2)**(n/2) / gamma(n/2+1)

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