Adopt declarative config (setup.cfg or pyproject.toml)

[jaraco]

I am also aware that it is not recommended to have a large amount of code written in setup.py as it is now. I confess that I do not know much how to properly replace (or abolish) things like class test(Command) and class post_install(install) when adopting more modern ways such as pyproject.toml or setup.cfg.

Originally posted by @junkmd in #216 (comment)

[jaraco]

We can incrementally move metadata from setup.py to setup.cfg and decide if the remaining features in setup.py are needed. We can also use this as an opportunity to modernize other packaging concerns (like declaring the non-legacy setuptools backend).