Log Message:
Re-import py-graphviz-1.3.1 as graphics/py-graphviz.

Was previously in py-pygraphviz.

Fixes PKGNAME to match pkgsrc conventions.
While here, fix build by adding pkg-config tool and minor cleanup.

PyGraphviz is a Python interface to the Graphviz graph layout
and visualization package. With PyGraphviz you can create,
edit, read, write, and draw graphs using Python to access the
Graphviz graph data structure and layout algorithms. PyGraphviz
is independent from NetworkX but provides a similar programming
interface.