Change shell to python, where suitable

[zmiklank]

Some steps in the action, e.g.:

    - name: Generate tmt variables
      id: generate_tmt_vars
      run: |
        python -c 'import json; print({} if not "${{ inputs.variables }}".strip() else json.dumps({key: value for key, value in [s.split("=", 1) for s in "${{ inputs.variables }}".split(";")]}))' > variables
        echo "::set-output name=TMT_ENV_VARS::$(cat variables)"
      shell: bash

use python commands, but bash shell, which makes them hard-to-read.
I propose to change the shell to python and execute python commands one by one.
This should be possible, as Github Actions offer python as one of its shells[1].
It would be nice to have this covered with tests too.

[1]https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idstepsshell

[phracek]

Another fix is #49

[phracek]

We will use TypeScript soon. This is not valid anymore.