Provide consistent story on dependencies

[woodt]

Currently there are two unrelated sets of dependency information in the repo:

• pyproject.toml
• requirements.*

Main issue is they are not in sync; secondary issue is that the custom scripts used to maintain the requirements.txt file are not a well-known pattern, and probably deserve a mention in the README.

Ideally, there would be one source of truth. Recent versions of pip seem to deal with installing dependencies successfully from the pyproject.toml file, so in theory we don't need the requirements files at all.

Work on this issue should clean up the requirements story so that:

(1) there is one source of truth
(2) if we need alternate representations of the dependencies (e.g., a requirements.txt file) they should be generated from the source of truth.