Please depend on `igraph` instead of `python-igraph`

[szhorvat]

Please depend on igraph instead of python-igraph, and suggest using pip install igraph in diagnostic messages. See https://github.com/igraph/python-igraph/issues/699 for an explanation. Places in this package that might need an update:

https://github.com/search?q=repo%3Admulyalin%2FN2G%20python-igraph&type=code

This is one of the most popular packages on PyPI that still hasn't switched to depending on igraph instead of python-igraph.