Implement coverage aggregation

[jaraco]

This post by Hynek describes a technique for aggregating coverage results across independent jobs. Let's explore applying that technique to skeleton-based projects.

[johnslavik]

I'm already doing this in my projects.
https://github.com/bswck/slothy/blob/main/.github%2Fworkflows%2Fcoverage.yml

I can work on this as soon as I get a laptop, which is in a few days ^^