Improve packaging

[eggplants]

I introduce useful packaging way to use setup.cfg, setup.py, and pyproject.toml.

It will be able to use both pip install . and pip install -e ., and also write almost configurations of tools in setup.cfg.

When you want to install dev dependencies, you can do it by running pip install .[dev].

[aucampia]

we should be uploading wheels also, but fine as is as it already improves on previous commands.

[aucampia]

I'm fairly sure this is deprecated:

• setup.py test is going to be deprecated pytest-dev/pytest#5534
• https://docs.pytest.org/en/latest/explanation/goodpractices.html#integrating-with-setuptools-python-setup-py-test-pytest-runner

  Integration with setuptools is not recommended, i.e. you should not be using python setup.py test or pytest-runner, and may stop working in the future.

Maybe it is okay for now, but better to point people to tox if they want something easy to manage.

I'm somewhat okay with the change you made as is, because it makes it no worse, but this is something we have to fix sometime.

[aucampia]

Change looks good, I'm a bit on the fence on removing the requirements.txt files, but I can't really see a good case for keeping them. I'm open to hearing other opinions, worse case scenario we can add them back but I think it is best to not use requirements.txt to populate the dependencies so either way your change to setuptools keywords (now in setup.cfg) is for the better.

[eggplants]

reverted deletion of requirements.txt.