Update networkx to 2.4

Changelog:

Highlights of this release include:

• Remove deprecated code from 1.x
• Support for Python 3.8
• Switched to pytest for testing
• Last release to support Python 3.5

A complete changelog can be found at https://networkx.github.io/documentation/networkx-2.4/release/release_2.4.html

run examples in /usr/share/doc/networkx-2.4/examples/basic/, should see network topology plots in graphs.