Add support for graphviz-py

[Alwinator]

graphviz-py makes it possible to run Python code inside of Graphviz diagrams, as well as using variables. I extended asciidoctor-diagram to support it.

[pepijnve]

Looks good to me. Thanks for the contribution!

[Alwinator]

@pepijnve You are welcome! Thanks for approving my code!

[pepijnve]

I had a look at your graphviz-py project in detail this morning. I'm wondering if an extra integration is actually necessary here. Since you support the same CLI interface as dot you could also just point the existing graphviz integration to graphviz-py. You can do that by setting the dot document attribute (e.g. pass -a dot=/usr/bin/graphviz-py to the Asciidoctor CLI).

[Alwinator]

@pepijnve Thank you so much for the reminder! I forgot to add an argument for variables. This makes it possible to calculate the same diagram with different values. I think that is very convenient.

[graphviz_py, "graphviz_py_example", "SVG", argument="foo=5;bar=7"]
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include::my_diagram.py.dot[]
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[graphviz_py, "graphviz_py_example", "SVG", argument="foo=24;bar=32"]
----
include::my_diagram.py.dot[]
----

[Alwinator]

From my side, it is ready for merging now!

[Alwinator]

@pepijnve Thank you for merging! I am looking forward to the new version :)

[pepijnve]

I'm picking up maintenance work again after a bit of a hiatus. A new release should be ready later today.

[Alwinator]

@pepijnve Thank you! That is great!