Underground

Misc packaging updates.

- Drop python 3.6
- Pin requires a little more tightly
- Misc fixes to the CI setup
- Rename master->main
- Remove old docs from readme
- Migrate away from `python setup.py sdist`

Add support for detecting stalled trains (WardBrian )

- Bump pydantic version

- Ignore empty departure times from MTA.

Pin dependency versions, add testing support for py versions up to 3.10

- Adopt new MTA API (https://api.mta.info/), no longer support the old API. (fixes #19 ) (https://datamine.mta.info/). Underground users may now either provide a full URL to their feed, or a train route. If a route is provided, the correct URL will be determined by underground.
- Migrate from poetry install flow to the usual setup.py. Change CICD and tox pipeline appropriately. (fixes 20)
- Use a well-maintained request mock instead of my request hack.
- Move the pypi push to a separate workflow.
- Isolate the package within a `src/` directory. (fixes 21 )
- Remove unittest.mock imports and use pytest monkeypatch instead