Dijkstar

In progress...

- Optimized `Graph.edge_count` property (elliotwutingfeng).

- Dropped support for Python 3.6 and 3.7 as these version are EOL (see
https://devguide.python.org/versions/#versions). These versions are no
longer tested, but Dijkstar should continue to work with them for the
time being.
- Added nominal support for Python 3.10, 3.11, and 3.12. No code changes
were made to support these versions--they were just added to the list
of versions tested using `tox`.
- Fixed import of `MutableMapping`. This was fixed a while back but
never released.
- Switched from `flake8` to `ruff` for linting and reformatting code.
- Made a couple tweaks to support the latest version of Starlette
(currently 0.31.0). In particular, `lifespan` is now used instead of
`on_startup`.

Changes for this verson weren't recorded.

- Updated `setup()` with `include_package_data=True` to ensure HTML
templates are included in distributions and installed.
- Upgraded Starlette 0.12 => 0.13.
- Fixed some `serve` script flags (again).
- Added `--root-path` to `serve` script. It's used to set the ASGI root
path so that, for example, URLs will be generated with the correct
prefix when the server is running under a path other than /.
- Fixed URLs in home template by using `url_for()`.

- Updated manifest to ensure HTML templates are included in
distributions.
- Fixed default values of some `serve` script app config args (the
default values for all app config args must be `None` so that if
they're not passed they'll be set from the environment).