Does not build with Sphinx 3.1: Theme error: An error happened in rendering the page morse.

Reported here:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=963651

Seems to be caused by:
````
> Theme error:
> An error happened in rendering the page morse.
> Reason: UndefinedError("'warn' is undefined")
> make[4]: *** [CMakeFiles/doc.dir/build.make:60: CMakeFiles/doc] Error 2
````