Constraint networkx to <2.3 for Python 2
Constraint networkx to <2.3 for Python 2

NetworkX released 2.3, dropping support for Python 2.

https://github.com/networkx/networkx/blob/master/doc/news.rst

Depends-On: https://review.openstack.org/#/c/652094

Change-Id: I59afe81d7868605434d2a42658c5b09a6a546732

switch-ch/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder,openstack/diskimage-builder