Require `python-graphviz`
Require `python-graphviz`

While we don't strictly need `python-graphviz`, it is convenient to have
`python-graphviz` if we ever want to visualize any of the Dask graphs
behind Dask objects. This need comes up often enough that it is better
to just require `python-graphviz`. Hence we add `python-graphviz` to our
run requirements for the workflow. This should also aid in any
demonstrations that we may wish to do with the workflow as well.

DudLab/nanshe_workflow,nanshe-org/nanshe_workflow