add get_graph_dot_code and show_dot (#151)

* add visualize_graph

* add package

* replace graphviz

* lint fixes

* small style fixes

* fix exception text

* restructure

* add a small test

* skip test if dot not available

* further restructure, rename to get_graph_dot_code

* remove useless assert

* small doc fix

* change Iterable to Collection

* flake8

* fix log message

* explicit return in non-svg case

* make sure that nodes not in graph.keys() appear in output

* rename dot.py -> graphviz.py

* add foldmethod

* add edge_labels

* fix doc

* trigger GitHub actions

* bump CI again

* rename to as_graphviz_dot and add noed_labels

* remove explicit edges storage

* test node_labels explicitly

* flake8