FIX OvR with constant label for non-predict methods
FIX case where label is constantly absent
TST use assert_warns and modernise test for constant predictor
FIX avoid using sequences of sequences and fix tests
Merge pull request #3308 from jnothman/ovr_constant_predict_proba

[MRG] FIX OvR with constant label for non-predict methods
DOC hmm example has been removed
FIX Anisotropic hyperparam optimization of GP bounds

Fixes #2798, #2867.
DOC remove reference to the bootstrap target
DOC remove spurious plt.clf() from ROC example
DOC add missing plt.figure() for multiplot support in ROC example
DOC Document return of fetch_olivetti_faces
ENH sparse matrix support in label binarization
FIX Forced dtype to int when densifying array in label_binarize