Merge setup.cfg into pyproject.toml

[nchammas]

There is a lot of churn in the Python packaging ecosystem, hopefully for the better in the long term. But for now, it means more busywork.

setup.cfg is apparently going to be deprecated at some point, and all of its functionality subsumed into pyproject.toml. That means we will need to move the dynamic versioning plus all the settings for pytest and flake8 from setup.cfg to pyproject.toml.

I didn't do it as part of the Flintrock 2.1.0 release (#369) because setup.cfg is still a ways out it seems from being deprecated, and because there are still several open discussions about the details of how and when pyproject.toml will absorb it.

So there's no rush to do this. When we do want to pick up this work, we'll want to refer to these documentation pages and issues:

• https://packaging.python.org/en/latest/guides/single-sourcing-package-version/
  • Guide for "Single sourcing the package version" refers to setup.py pypa/packaging.python.org#1260
  • WIP: "modernize" single-source versioning pypa/packaging.python.org#1273
• Eventually deprecate setup.cfg with automatic conversion to pyproject.toml pypa/setuptools#3214