Remove matplotlib warnings from examples executed when generating the documentation

There is a warning generated in our documentation for the 2 following examples:

The warning is linked with the decoration of the yticks:

/home/circleci/project/examples/inspection/plot_permutation_importance.py:133: UserWarning: FixedFormatter should only be used together with FixedLocator
  ax.set_yticklabels(feature_names[sorted_idx])

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