Support the Pipenv package manager

[edmorley]

The classic Python buildpack currently supports the package manager Pipenv: https://pipenv.pypa.io https://github.com/heroku/heroku-buildpack-python/blob/main/bin/steps/pipenv-python-version https://github.com/heroku/heroku-buildpack-python/blob/main/bin/steps/pipenv

We should decide whether we want to still support it in the CNB, or whether Pipenv's declining usage (and mixed stability/issues upstream) mean we would rather only support pip + Poetry (+ ideally uv) instead.

Internal tracking epic