Make _PairsClassifierMixin inherit from ClassifierMixin ?

[wdevazelhes]

I just realized from here https://scikit-learn.org/stable/modules/cross_validation.html#computing-cross-validated-metrics, that "When the cv argument is an integer, cross_val_score uses the KFold or StratifiedKFold strategies by default, the latter being used if the estimator derives from ClassifierMixin."
So we would maybe like _PairsClassifierMixin to inherit from ClassifierMixin to have a nice stratification based on the class when doing cross-validation. We could also maybe add an explanatory paragraph about that in the docs.
Note however that ClassifierMixin implements a default score method that is accuracy. I guess it's OK to overwrite it with our own default score (roc_auc) in the end, but it's good to clarify that.