Add support for pyproject.toml

[jylin]

The same functionality should work, but you would just have to skip to only the "[dependencies]" section in pyproject.toml.

[janusheide]

I would also really like to see support for pyproject.toml in pur as I really like the development cycle pur enables, and is using / have used it in a bunch of projects.

Since many things have changed wrt. how dependencies are managed in Python, I am not sure that supporting pyproject.toml in pur necessarily makes sense? Anyway I wrote a small spiritual successor to pur you can check it out here: https://pypi.org/project/uppd/.