pip install -r requirements.txt in Python feature

I see #40 gave `installJupyterlab` to the Python feature.

Would you accept a PR to more broadly allow dependencies to be installed via pip's [`requirements.txt`](https://pip.pypa.io/en/stable/reference/requirements-file-format/) file?

Either optimistically installing if present (which would be a behavior change)
and/or by adding an e.g. `installRequirements` option to enable / disable this behavior?

Start with the Python feature and the existing installJupyterlab option mentioned in #40, then review pip's requirements.txt format linked in the issue. Compare the alternatives of automatic installation versus an installRequirements option, and define which behavior and configuration should be accepted before implementation. Done means the chosen requirements.txt behavior is documented and verified for the Python feature.

Scritto dal modello di indicizzazione a partire dal testo della issue.