calculate studentized residuals
sigma : None or float
estimate of the standard deviation of the residuals. If None, then the estimate from the regression results is used.
stzd_resid : ndarray
studentized residuals are defined as
resid / sigma / np.sqrt(1 - hii)
where resid are the residuals from the regression, sigma is an estimate of the standard deviation of the residuals, and hii is the diagonal of the hat_matrix.