[issue27739] add math.sign/signum

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

copysign() is part of the standard C maths library (that is why it was added to the math module).

sgn/sign/signum was used in old languages that didn't have copysign() (Basic, Pascal, etc).

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<http://bugs.python.org/issue27739>
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