Change MRO

[jtilly]

Closes #878.

In CI against the latest scikit-learn version, we're seeing

FAILED tests/glm/test_glm.py::test_check_estimator[GeneralizedLinearRegressor-kwargs0] - AssertionError: GeneralizedLinearRegressor is inheriting from mixins in the wrong order. In general, in mixin inheritance, more specialized mixins must come before more general ones. This means, for instance, `BaseEstimator` should be on the right side of most other mixins. You need to change the order so that:
RegressorMixin comes before/left side of BaseEstimator
FAILED tests/glm/test_glm.py::test_check_estimator[GeneralizedLinearRegressorCV-kwargs1] - AssertionError: GeneralizedLinearRegressorCV is inheriting from mixins in the wrong order. In general, in mixin inheritance, more specialized mixins must come before more general ones. This means, for instance, `BaseEstimator` should be on the right side of most other mixins. You need to change the order so that:
RegressorMixin comes before/left side of BaseEstimator

This check was introduced with scikit-learn/scikit-learn#30234.

Was there are reason we ordered things they way we did or is this safe to change?