Fix GraphViz + pygraphviz integration

[stevenjkern]

pygraphviz has recently become un-pip installable in continuous integration (and for me locally on OSX and linux). Some effort will be needed to figure out what the correct procedure for installing these libraries should be to reproducibly produce an environment with both installed. The CI recipes should also have entries in the testing matrix that have GraphViz+pygraphviz installed and not installed for each Python version and platform.