Add build job, more declarative package metadata

[bollwyvl]

A couple things here:

• as per add remaining test data #81, shows a CI approach that primarily tests the code-as-distributed
  • an up-front build job (with no matrix)
  • builds the distribution artifacts
  • runs the linter
  • tests code-in-source will keep the coverage file names the same
  • uploads coverage
  • uploads the artifacts
  • the test matrix job:
    • downloads the artifacts
    • picks one to install (kind of annoying, because platform differences, and globs on windows)
    • runs the tests
• In working that up, moved a lot of the static (and dynamic) setup.py stuff to setup.cfg, as modern (last two years or so) setuptools does a lot more of the heavy lifting now, e.g. worrying about encodings, etc.
  • it doesn't do requirements.txt, but it may be worth moving those to the canonical install_requires
  • tests_require could also be added... but pip won't install extras from a local artifact, so it still can't be a single source of truth without weird gotchas

[klieret]

Thank you so much ❤️ I will take a look at it tomorrow, sorry for being a bit slow (also with the RandomFileTree pull requests)

[klieret]

Took me some time to read through all the changes, but it's great stuff (and I learned a lot)!

[klieret]

tests_require could also be added... but pip won't install extras from a local artifact, so it still can't be a single source of truth without weird gotchas

Hmm, we might get this to work with a combination of this and this though, right?