BUG: RLM bcov_unscaled was broken by 61175498ada40061c97caf89b213997d36378f7d
BUG fix import for python 3
TST: add generic tests across models (forgotten to commit)
REF/ENH: self.use_t in summary, additional robust sandwiches, reset df_resid
BUG: regression robust cov, fix and transmit `use_correction`
TST: OLS with cluster robust, first test class (4 failures)
REF: t_test uses model.df_resid not results.df_resid, adjust both
TST: fix tests
REF: add use_t to RegressionResults pvalues and confint
TST: cluster robust against ivreg2 "large" except fvalue
TST  test for OLS cov_nw_groupsum
TST: add test for nw_panel, default still df need changing
TST: clean up results_grunfeld file, duplicate, typo
TST: add cov_cluster_2groups tests against ivreg2, "small" is skipped
TST enable cov_cluster_2groups_small with low precision
REF: don't use `.model.df_resid` in results, see #1203
     make it a simple attribute in RegressionResults
REF: RegressionResults convert HCx to cache_readonly, closes #1206
DOC: adjust docstring for cov_HC# and HC#_se for previous commit, see #1206
DOC  cov_type version in docstring for get_robustcov_results
DOC: docstring for f_test, wald_test (not DRY, strange changeset)