Use YAML template for repeating sections in CI

[nikosavola]

The steps such as

      - uses: actions/checkout@v2
      - uses: actions/setup-python@v2
        with:
          python-version: '3.9'
      - uses: actions/cache@v2
        with:
          path: ~/.cache/pip
          key: ${{ runner.os }}-${{ env.pythonLocation }}-pip-${{ hashFiles('**/setup.py') }}
          restore-keys: |
            ${{ runner.os }}-${{ env.pythonLocation }}-pip-

should be turned into a YAML template. Alternatively, only the caching could be shortened.

[nikosavola]

Seems that YAML anchors are not currently supported, see https://github.community/t/support-for-yaml-anchors/16128/51