Pin python-igraph to latest version 0.8.3

[pyup-bot]

This PR pins python-igraph to the latest release 0.8.3.

Changelog

0.7.0

See release notes: http://igraph.org/2014/02/04/igraph-0.7-python.html

Links

• PyPI: https://pypi.org/project/python-igraph
• Changelog: https://pyup.io/changelogs/python-igraph/
• Homepage: https://igraph.org/python

[coveralls]

Coverage increased (+0.06%) to 93.9% when pulling d805b75 on pyup-pin-python-igraph-0.8.3 into 3e7acba on master.