WIP reworking pyproject.toml so that pytest unit tests can be run

[ceteri]

Reworking some of the details for PR 300 since GH got that potential merge locked into Merge Hell.

• pyproject.toml is now based on dynamic metadata so that requirements.txt can continue as the ground truth for dependencies
  • https://stackoverflow.com/questions/73600082/how-to-reference-a-requirements-txt-in-the-pyproject-toml-of-a-setuptools-projec
  • https://setuptools.pypa.io/en/latest/userguide/pyproject_config.html#dynamic-metadata

Let's work on this in stages:

1. migrate to using pyproject.toml and setup.cfg
2. pipenv support
3. poetry support

I found that the poetry build was interacting to prevent pytest unit test from being able to be run, so I've redacted that, falling back to using setuptools for the build (for now).

Descriptions which were specific to poetry have been copied into the for_poetry subdirectory.