CI: Added python versions check

[KClaudius]

No description provided.

[andizimmerer]

General note on limitations in GitHub actions:

• Using a matrix is cool and convenient. great.
• GitHub does not allow individual "success badges" per matrix - only on job level.
• Hence, the current workflow will allow you to only show a single badge in the README basically saying "yeah some python versions are supported - go figure it out yourself".
• This is fine, but then at least make it very explicit in the README.
• What I personally would prefer, even though this results in duplicate code: copy the workflow into 3 different files, each handling exactly one python version. Then we can have a badge for every supported Python version with the version in the name of the badge. I think this is much more readable and clearer; and I personally would sacrifice code-duplication for having this. But it's your call on how you want to do that. Both has advantages.

[andizimmerer]

I would rather call it "Python Version Check"

[andizimmerer]

let's add

  workflow_dispatch:

here as well. This allows triggering the run explicitly from the GitHub UI. Might be useful here (for formatting probably not so much)

[andizimmerer]

let's also add something like

concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.head_ref || '' }}-${{ github.base_ref || '' }}
  cancel-in-progress: ${{ !contains(github.ref, 'main')}}

This cancels workflow runs on subsequent pushes (except on main). I useful for expensive jobs. I guess it's not super expensive, but then you also learn how to do that ;)

[andizimmerer]

please add a note that exactly these three versions are supported.