python-igraph was renamed to igraph

[szhorvat]

The python-igraph package was renamed to simply igraph. Support for the earlier name is stopping. See https://pypi.org/project/python-igraph/

Please update the references in plotly.py:

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

This is basically changing all the pip install python-igraph to pip install igraph.

[alexcjohnson]

Thanks @szhorvat

@LiamConnors this looks like it only impacts the docs.