- Dropped support for Python 2.7, 3.4, and 3.5.
- When adding a node with neighbors, nodes are now created for neighbor
nodes that aren't already present in the graph.
- Likewise, when adding an edge `(u, v)`, a node is added for `v` if
it's not already present in the graph.
- Added `dijkstar` console script.
- Added an HTTP graph server based on Uvicorn and Starlette. Added
corresponding `serve` subcommand to `dijkstar` console script.
- Add RunCommands dependency to make defining console scripts and dev
commands easier.