It seems that the sklearn classes are not being found during the sphinx-build CI test.

<unknown>:1:py:class reference target not found: sklearn.svm.classes.SVC

See:
https://travis-ci.org/wikimedia/revscoring/builds/586781889#L1246

The temporary workaround is to add the full path to these classes to nitpick_ignore as seen in this PR:
https://github.com/wikimedia/revscoring/pull/451/files#diff-85987f48f1258d9ee486e3191495582dR110

A potentially helpful link:
https://stackoverflow.com/questions/46080681/scikit-learn-intersphinx-link-inventory-object