Fix app.warn, deprecated in Sphinx 1.8.5 and removed in Sphinx 2.0
Fix app.warn, deprecated in Sphinx 1.8.5 and removed in Sphinx 2.0

The function `app.warn` was deprecated and has now been removed in
Sphinx 2.0.

djungelorm/sphinx-tabs,djungelorm/sphinx-tabs,djungelorm/sphinx-tabs