[issue40855] statistics.stdev ignore xbar argument

[Matti]

Matti <matti.jansson at fysik.su.se> added the comment:

If we estimate the mean using a sample we loose one degree of freedom so it will be divided by N-1, while if we have the mean independent of the sample it should be divided by N to be unbiased. 

i.e. 
example 1 
sqrt(((1-1.5)²+(2-1.5)²)/(2-1)) = 0.7...
example 3
sqrt(((1-1.5)²+(2-1.5)²)/(2)) = 0.5

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<https://bugs.python.org/issue40855>
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