* Migrated sphinx theme to pydata.
* Improved documentation, code style, `flake8` errors.
* Exposed some missing classes:
* `DirectedGraph`;
* `UndirectedGraph`;
* `dotstr_to_html`;
* `enrich_kwargs`;
* `run_graphviz`;
* `make_node_automaton`.
* Bug fix `*_copy`, `GraphView.*_degree`.
* The Graphviz-related function now correctly forward the kwargs arguments (e.g. `engine=`):
* `ipynb_display_graph`;
* `graph_to_html`;
* `dotstr_to_html`;
* `write_graphviz`;
* `run_graphviz`.
* `pybgl.AssocPropertyMap` can now only wrap a `defaultdict` (not a `dict`).