Dijkstar

- 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.

- Fixed import of `MutableMapping`, which was moved from `collections`
to `collections.abc` in Python 3.3 (xSetech)
- Added nominal support for Python 3.9

- Started supporting for Python 3.7 and 3.8 (no code changes required).
- Added support for undirected graphs. The implementation is simple. An
undirected graph is just a directed graph that automatically adds the
edge `(v, u)` when `(u, v)` is added and automatically removes
`(v, u)` when `(u, v)` is removed.
- Added several new `Graph` methods and properties:
- `get_data()` -> Get underlying data dict
- `node_count` -> Return the number of nodes in the graph
- `get_node()` -> Get a node
- `remove_node()` -> Remove a node
- `edge_count` -> Return the number of edges in the graph
- `get_edge()` -> Get an edge
- `remove_edge()` -> Remove an edge
- `subgraph()` -> Get a subgraph with the specified nodes
- `guess_load()` -> Load any supported graph type from file
- `__eq__()` -> Graph equal to other graph? (compares data dicts)
- `__repr__()` -> Graph representation (repr of data dict)
- Made `Graph`'s mapping interface methods (e.g., `__setitem__()`) call
the corresponding method (e.g., `add_node()`) instead of vice versa.
- Made `Graph.add_node()` return the added node.
- Made `Graph.add_edge()` return the added edge.
- Fixed `Graph.add_edge(u, v)` so it *always* updates `v`'s incoming
list, both when `u` is already in the graph and when it's not.
- When a node's incoming list becomes empty, delete it (i.e., the node's
entry in the incoming list, not the node).
- In `single_source_shortest_paths()`, operate on the underlying data
dict instead of going through `Graph`'s mapping interface. This should
improve performance (or, at least, that's the intent).
- Added `debug` flag to `single_source_shortest_paths()`. When set,
additional debugging info is returned along with the path info.
- Improved README, esp. wrt. path-finding examples and cost functions.
- Added and improved docstrings.
- Added more tests.
- Added Travis CI config.
- Started using setuptools instead of distutils for packaging.
- Added `dijkstar.__version__`.
- Improved some other packaging-related stuff.

- Added option of passing an open file to `Graph` read/write methods (in
addition to being able to pass a path name as before).

- Added incremental count to priority queue entries in
`algorithm.single_source_shortest_paths()`. The reason for this is
given in the standard library docs for the `heapq` module: "The entry
count serves as a tie-breaker so that two [items] with the same
priority are returned in the order they were added. And since no two
entry counts are the same, the tuple comparison will never attempt to
directly compare two tasks."

- Updated project metaata:
- Changed project status from alpha to beta
- Updated author name & email
- Removed support for Python 2.6
- Added support for Python 3.4, 3.5, and 3.6

- Added flake8 config and removed lint

- Added tox config

- Bugfix: Pass previous edge to cost & heuristic functions instead of
the whole predecessor tuple (node, edge, cost).
- Add a test that passes a cost function to `find_path()`.
- Improve package metadata.