Test on newer Python versions

[jtpio]

Currently we test on the following Python versions:

https://github.com/plasmabio/tljh-repo2docker/blob/c42f08a0832a9abdd131be246d339df2c2ce00f3/.github/workflows/test.yml#L15

Since then 3.11 and 3.12 are now also available.

We probably don't need to test on all versions. Maybe testing on the oldest / newest would be enough, in that case 3.8 and 3.12.

[jtpio]

Labeling as "good first issue", in case someone would like to give it a try.

Hopefully simply updating the list is enough, and CI stays green!