Add graphics/py-graphviz, simple Python interface for Graphviz

This package facilitates the creation and rendering of graph descriptions in the
DOT language of the Graphviz graph drawing software (repo) from Python.

Create a graph object, assemble the graph by adding nodes and edges, and
retrieve its DOT source code string. Save the source code to a file and render
it with the Graphviz installation of your system.

Use the view option/method to directly inspect the resulting (PDF, PNG, SVG,
etc.) file with its default application. Graphs can also be rendered and
displayed within Jupyter notebooks (a.k.a. IPython notebooks, example) as well
as the Jupyter Qt Console.

WWW: https://github.com/xflr6/graphviz

Already existed a port named graphics/py-graphviz, it was moved in
ports r408353 to graphics/py-pygraphviz because it was using an
incorrect name. More info in the commit log [1].

1 - https://svnweb.freebsd.org/ports?view=revision&revision=408353

PR: 222205
Submitted by: lbartoletti@tuxfamily.org
Reviewed by: koobs
Differential Revision: D12319