Sunset implicit setup.py support

[edmorley]

Previously if an app had only a setup.py file and no Python package manager files (such as requirements.txt, Pipfile.lock, Poetry.lock or uv.lock), the buildpack would install the project using pip install --editable .

However, this implicit fallback doesn't make sense now that the buildpack supports multiple package managers, and so has to guess which package manager to use, and whether to install the project in editable mode or not.

As such, in #1897 this fallback was deprecated, and is now being sunset. This also brings the classic Python buildpack's behaviour in line with the Python CNB.

Apps that only have a setup.py file will now need to add an explicit requirements.txt file containing:

--editable .

That said, in general we recommend people don't use setup.py to declare their dependencies, since:

• the file is deprecated in favour of pyproject.toml
• it's intended more for libraries rather than applications, and it's much less common (and practical) to list all transitive dependencies in a setup.py, meaning apps using the file typically have unpinned dependencies, which is a production reliability risk. (Apps should either be using a package manager that supports lockfiles, or else using one of the pip requirements files substitutes for lockfiles, like pip-tools or pip freeze etc.)

GUS-W-19275444.

[edmorley]

https://devcenter.heroku.com/changelog-items/3509