This doc test failure also caught me by surprise at first.

The change is needed since the doc test creates labels as an ordinary python list (`labels = [1, 1, 2, 2]`). In the old versions of `LeaveOneLabelOut` and `LeavePLabelOut` (current master), `self.labels` was just a reference to the `labels` parameter, while in the new version of `LeaveOneLabelOut` and `LeavePLabelOut` (this pull request), `self.labels` is a numpy.array that is a copy of the given `labels` parameter.

As a consequence, the call to `print(lplo)` returns a slightly different result now, without commas (as usual for numpy.arrays)