upgrade networkx to version 1.8.1

[ohanar]

Source Tarball: networkx-1.8.1.tar.gz

Component: packages: standard

Author: R. Andrew Ohana

Branch/Commit: u/ohanar/networkx @ 32b4f50

Reviewer: Volker Braun

Issue created by migration from https://trac.sagemath.org/ticket/15532

[kiwifb]

comment:2

I thought this would break doctests but it doesn't seem to. That's something I had seen with networkx 1.7 I am sure.

[vbraun]

comment:3

lgtm

[vbraun]

Reviewer: Volker Braun