GraphArtist RTD build

[iosonofabio]

PR #665 seems to have broken the Readthedocs doc build, see here: https://readthedocs.org/projects/igraph/builds/20263052/

Any idea what's going on here @iosonofabio ?

Originally posted by @ntamas in #655 (comment)

[iosonofabio]

As I wrote over there, this is probably a corner case with empty graphs

[iosonofabio]

move discussion to the PR