Calculate the Wilcoxon signed-rank test.
The Wilcoxon signed-rank test tests the null hypothesis that two related samples come from the same distribution. It is a a non-parametric version of the paired T-test.
x : array_like
y : array_like, optional
z-statistic : float
p-value : float
Because the normal approximation is used for the calculations, the samples used should be large. A typical rule is to require that n > 20.