Set up a CI workflow for coverage measurement

[Technologicat]

Make tests run on 3.6, 3.7, 3.8 and PyPy3. Measure coverage, too.

unpythonic has suitably customized workflows that can be quickly adapted to this.

Also grab its flake8rc, and maybe tune it to remove some idiosyncracies.

Also try black; but may be it doesn't make sense on this codebase. A macro expander is almost as far from a regular Python project as language extensions such as unpythonic are. Having blank lines serve as logical paragraph breaks is valuable. Some of the PEP8 rules occasionally hinder that, causing blank lines (or double-blank-lines) to no longer map 1:1 to breaks between logical units. PEP8 is great as long as it improves readability, but foolish consistency and all that.

Depends on #2.

[Technologicat]

As of 853d1bf, CI enabled for running automated tests. We don't have many yet, but it's a start - and at least they now run automatically on all supported Python versions.

The flake8rc and pylintrc I'm using for linting during development are already in the repo.

We still need to set up codecov, like unpythonic has, and then add the badges to the README (see #7).

Steering away from black for now.

[Technologicat]

As of e5a678b, CI enabled for coverage measurement.