Bw-graph-tools

* Fix [18 - Duplication of values from first heuristic](https://github.com/brightway-lca/bw_graph_tools/issues/18)
* Update packaging
* Python 3.9 compatibility

* Remove `scikit-network` dependency

* Updated CI scripts

* Fix [8: `traverse_edges` returns when values below cutoff or excluded](https://github.com/brightway-lca/bw_graph_tools/issues/8)

* Fix scikit-network dependency as API changed in new version

* Refactor some testing functions