py-graphviz: updated to 0.13

Version 0.13

Add explicit support for layout engine escape sequences such as ``\l`` and
``\N``. These already worked implicitly before but where broken by backslash
escaping in ``0.12``, which is reverted by this release. Escaping now resembles
the stdlib ``re`` module: literal backslashes need to be escaped (doubled),
which is most conveniently done by using raw string literals for strings that
use escape sequences (including escaped backslashes), e.g. ``label=r'\\'``.

Add ``escape()`` function (resembling ``re.escape()``) for disabling all
meta-characters in a string for rendering.

Use ``logging`` in example notebook, add notebooks demonstrating layout engines
and escape sequence usage, improve tests with parametrization.

(adam)