Reduce duplication between pyproject.toml and setup.py

[anjakefala]

Since it seems like poetry cannot get what it needs from a standard setup.py, I will try to adapt and see if I can de-duplicate and put as much as we need into pyproject.toml.

(As an example of something I need, I really like installing in-development projects in an editable state, and it seems like to do that we need both pyproject.toml and setup.py? https://stackoverflow.com/questions/62983756/what-is-pyproject-toml-file-for.)

Originally posted by @anjakefala in #4 (comment)

[anjakefala]

poetry export --without-hashes --format=requirements.txt > requirements.txt