py-graphviz: updated to 0.16

Version 0.16
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Add ``.unflatten()`` method to ``Graph``, ``Digraph``, and ``Source``. Add
standalone ``unflatten()``.

Make ``Source.__str__()`` return the ``.source`` instead of the ``repr()``
(like ``Graph`` and ``Digraph``).

Render with ``dot -K<engine> ...`` instead of ``<engine> ...`` internally
(work around `upstream issue

Add documentation hint to archived upstream version for Windows.

Re-render most documentation graphs with Graphviz 2.44.1.

(adam)