Bw-graph-tools

* Fix [16](https://github.com/brightway-lca/bw_graph_tools/issues/16) - Add optional `max_depth` argument to graph traversal

Added the following attributes to `Node` dataclasses:

* direct_emissions_score_outside_specific_flows (float): The score attributable to *direct emissions* of this node which isn't broken out into separate `Flow` objects.
* remaining_cumulative_score_outside_specific_flows (float): The *cumulative* score of this node, including direct emissions, which isn't broken out into separate `Flow` objects.
* terminal (bool): Boolean flag indicating whether graph traversal was cutoff at this node

* 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)