This was missed in I90ee8a2081c2a0465441a8d81d161f4887b4e1fb
presumably because we no longer get warnings as errors, or
some such wickedness. Anyway, this fixes docs build using tox.
Change-Id: Ic8ec0c54a7c6502a0a4ea91d034fa9c1ae5e59d4
Closes-Bug: #1767192