consider moving to poetry from pipenv

[yashaka]

https://poetry.eustace.io/

[aleksandr-kotlyar]

https://python-poetry.org/

Also an interesting article: https://browniebroke.com/blog/migrating-project-to-poetry/

To not overwrite all the arguments to poetry: pydantic/pydantic#314
The most interesting comment: pydantic/pydantic#314 (comment)

Just to add my 2 cents about choosing a new build tool, and the differences between Flit and Poetry, from what I have found about all this.

I haven't even developed a personal preference yet, but as it was a bit confusing for me, here's a summary of what I have found.

• pipenv was developed with a file Pipfile (also in TOML format) to replace venv and requirements.txt at the same time. It was adopted and endorsed by PyPA (Python Package Authority, the guys that run PyPI).
• They wrote the PEP about pyproject.toml, it is the new "standard" file by PyPA). It's supposed to be an alternative comparable to setup.py and setup.cfg. Not to "deprecate" them necessarily, but to make them optional. As everything is declared in a simpler configuration file, it's supposed to centralize and simplify package configuration.
• Poetry and Flit both use the same pyproject.toml file, but in a slightly different way.
• Flit uses it as it was intended, and so it's endorsed by PyPA, to do project building and publishing. Nothing else.
• Poetry uses the same file to handle package building and publishing but also puts development dependencies in there. And it also handles development virtual environments. So, it intends to replace setup.py, setup.cfg but also Pipfile (and with that requirements.txt). And to replace venv and pipenv as commands. More like npm does in the JavaScript world. Everything is done with npm and there's a single package.json file.
• Poetry creator and PyPA guys even had some arguments about all the subject.
• Flit seems to be more "standard" and "endorsed", but Poetry seems to be more popular.
• With Flit, the pyproject.toml file would only be replacing setup.py and setup.cfg, but Flit wouldn't care if you use requirements.txt, Pipfile` or anything else for development. It would only care about package building.

[aleksandr-kotlyar]

TODO

1. Add poetry to project
2. Move dependencies to pyproject.toml
3. Update README and update CONTRIBUTING about poetry usage to contribute to selene.

[aleksandr-kotlyar]

Resolved in #302