GitHub Actions add possibility for caching python environments

[kbalog]

Describe in template-project README how it needs to be configured

[kbalog]

Depends on #42

[NoB0]

This is automatically done in the workflow now that we use pip and requirements.txt.

- name: Set up Python 3.9
      uses: actions/setup-python@v4
      with:
        python-version: 3.9
        cache: "pip"