Several places in Sage refer to graphviz.

When building from source, graphviz can make use of various optional dependencies: https://graphviz.org/download/source/
It seems too tricky to make it a normal package.

Current versions of graphviz seem to be licensed under the Eclipse Public License 1.0, which is a free software license but GPL-incompatible. https://gitlab.com/graphviz/graphviz/-/blob/master/LICENSE
So probably we should not make it a standard package.

Thus, we add it as a dummy script package only, so that we can provide system package information (https://repology.org/project/graphviz/versions).

We also add pygraphviz as a pip package - as part of Meta-ticket #31176 (Add packages from 3-manifolds / computop)

CC: @dimpase @dcoudert @seblabbe @NathanDunfield @culler @slel @videlec

Component: packages: optional

Author: Matthias Koeppe

Branch/Commit: ea35795

Reviewer: David Coudert

Issue created by migration from https://trac.sagemath.org/ticket/31177