add GHA CI configuration

[RalfJung]

Fixes #26

I also bumped the Ubuntu versions we test against to not be quite so ancient. Note that the latest Ubuntu LTS (22.04) fails, so it is commented out. It shows this error while "Running setup.py install for systemd":

       (tree fragment): In function ‘PyInit_reader’:
      (tree fragment):161:38: error: ‘PyTypeObject’ {aka ‘struct _typeobject’} has no member named ‘tp_print’
      (tree fragment):166:40: error: ‘PyTypeObject’ {aka ‘struct _typeobject’} has no member named ‘tp_print’
      (tree fragment):174:43: error: ‘PyTypeObject’ {aka ‘struct _typeobject’} has no member named ‘tp_print’
      (tree fragment):182:44: error: ‘PyTypeObject’ {aka ‘struct _typeobject’} has no member named ‘tp_print’
      (tree fragment):188:56: error: ‘PyTypeObject’ {aka ‘struct _typeobject’} has no member named ‘tp_print’
      (tree fragment):191:56: error: ‘PyTypeObject’ {aka ‘struct _typeobject’} has no member named ‘tp_print’

[The-Compiler]

Thanks! This is pretty cool. I added a couple of comments to hopefully simplify/improve some stuff.

[The-Compiler]

The error you had mentioned looks like this: mosquito/cysystemd#36

A comment there recommends doing pip install cysystemd instead, and indeed the PyPI page for systemd actually links to the cysystemd repository. So maybe changing that in run.sh would make things work?

[RalfJung]

Maybe? I'd say that is a separate change. My goal here is just to port the existing CI, not to improve/update the things we test. :) I had to change to newer Ubuntu releases since the old ones are no longer available or stopped working with recent pip packages, but I don't plan to do any more here than is necessary to maintain the old level of test coverage.

[The-Compiler]

Is there really a good reason to use Docker manually at this point? If I remember correctly, that was mainly a Travis CI thing? Couldn't we just use runs-on: ${{ matrix.dist }} above and run the script directly instead (probably after running setup-python).

Btw, to try those things out locally, there's act, in case you weren't aware of it yet.

[RalfJung]

Good question. As I just wrote in the other thread, I wanted to do the bare minimum of work to get CI green again. So this was the easiest thing to do. Feel free to simplify the setup in follow-up changes. :)