Update python-igraph dependency to igraph

[szhorvat]

I notice that even though this is recently created GitHub repo, it contains a dependency on the long-deprecated python-igraph package name. The updated package name is igraph.

https://github.com/search?q=repo%3AKellisLab%2Fscdemon%20python-igraph&type=code

For an explanation, please see igraph/python-igraph#699

[cboix]

Fixed, thanks for letting us know.