Graphviz

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Tag Python 3.8 support.

Fix quoting for non-ASCII numerals.

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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.

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Fix missing escaping of backslashes, e.g. in labels (pull request DNGros).

Add ``quiet`` argument to standalone ``view()`` function, and ``quiet_view``
argument on ``.render()`` and ``.view()`` methods. Suppresses the ``stderr``
output of started viewer processes (unavailable on Windows).

Add basic debug logging via the stdlib ``logging`` module.

Reformatted some examples, improved tests by using autospec for mocks.

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Include ``stderr`` in ``str()`` of raised ``subprocess.CalledProcessError``.

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Add ``quiet`` argument to ``.render()`` and ``.pipe()`` methods of ``Graph``,
``Digraph``, and ``Source`` objects, allowing to suppress ``stderr`` of the
layout subprocess (parity with stand-alone ``render()`` and ``pipe()``
functions).

The rendering process for ``render()`` methods and stand-alone function is now
started from the directory of the rendered dot source file. This allows to
render graph descriptions that use relative paths inline (e.g. for referring to
image files to be included) by using paths relative to the source file
location. Previously, such relative paths would need to be given relative to
the directory from which ``render()`` was started, so this change is backwards
incompatible for code that relied on the previous behaviour.

Drop Python 3.4 support.

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Fix broken renderer argument in ``pipe()`` method and function.