try GHA without explicit python install

[pascal-sauer]

No description provided.

[tonnrueter]

Seems to work 🤷 Some notes:

• In order for this to be propagated to all other PIAM projects we'd need to edit inst/extdata/check.yaml

• This section

  - uses: actions/setup-python@v5
   with:
     python-version: 3.9
  
  - name: Install python dependencies if applicable
   run: |
     [ -f requirements.txt ] && python -m pip install --upgrade pip wheel || true
     [ -f requirements.txt ] && pip install -r requirements.txt || true

  oughta go since from what I understand this installs Python dependencies defined in a requirements.txt file in a Python v3.9 if said file is present in the repo which seems unnecessary for our R-based ecosystem. Also Python v3.9 seems kinda arbitrary since we (REMIND) requires >=3.11

• The pre-commit run --show-diff-on-failure --color=always --all-files section will run regardless since the GHAs run in a Ubuntu instance that comes with Python (albeit of unknown version). It uses the inst/extdata/pre-commit-config.yaml file to enforce useful coding etiquette standards

[tonnrueter]

Looks good except for inst/extdata/pikPiamPackages. Can't tell if that's how it oughta be