docs: def deriv(self, mu):
          """
          Derivative of the variance function v'(mu)
          """
          from statsmodels.tools.numdiff import approx_fprime_cs, approx_fprime
          # return approx_fprime_cs(mu, self)  # TODO fix breaks in `fabs
          # TODO: diag is workaround problem with numdiff for 1d
          return np.diag(approx_fprime(mu, self))