Fix GraphViz + pygraphviz integration

`pygraphviz` has recently become un-`pip install`able 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.