I forgot to change this in ffee547 as part of PR #1867
no unittests for the use_t option in cluster robust standard errors
(I think I only test that the attribute is set, but not that all results are verified in this case.
there should be a small sample test case, but maybe it's not yet used when cov_type is chosen in the fit method.)
found while looking at a cluster robust GLM-Poisson with use_t
(conf_int and hypothesis tests are correctly changed)
BUG: LikelihoodModelResults.conf_int use df_resid_inference closes #1903