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* fixed scikit-learn 0.22+ and 0.24+ support.
* allow nan inputs in permutation importance (if model supports them).
* fix for permutation importance with sample_weight and cross-validation.
* doc fixes (typos, keras and TF versions clarified).
* don't use deprecated getargspec function.
* less type ignores, mypy updated to 0.750.
* python 3.8 and 3.9 tested on GI, python 3.4 not tested any more.
* tests moved to github actions.