Add OneVs{One,All}Classifier._pairwise: fix for #7306 (#7350)

doc/whats_new.rst

sklearn/model_selection/_split.py

sklearn/model_selection/_validation.py

sklearn/model_selection/tests/test_split.py

sklearn/multiclass.py

sklearn/svm/base.py

sklearn/tests/test_multiclass.py

sklearn/utils/metaestimators.py

sklearn/utils/multiclass.py

sklearn/utils/tests/test_multiclass.py