Merge pull request #1200 from python-hyper/requires-io-master

[requires.io] dependency update on master branch

Author: alexwlchan

tox.ini

@@ -27,7 +27,7 @@ commands =
 
 [testenv:graphs]
 basepython=python2.7
-deps = graphviz==0.12
+deps = graphviz==0.13
 commands =
     python visualizer/visualize.py -i docs/source/_static