ci: report coverage

[blueyed]

I think CI builds should report coverage (for code and tests), e.g. to codecov.io.

It might be as easy as using py-cov as a toxenv to enable it, and then upload it from there.

For performance reasons it should probably not done for all builds.

I'd be happy to provide a PR for it, but wanted to ask about / discuss it first.

[JukkaL]

This would be nice if it can be implemented somehow without slowing down CI builds (e.g. if the coverage build happens asynchronously and doesn't affect build pass/fail state). We considered collecting coverage for all builds but the slowdown wasn't worth it.