SPDX licenses

[mauritsvanrees]

We should probably start using the the SPDX license expressions in pyproject.toml files in Plone packages. This needs setuptools 77 or higher, at least when creating the source dist or wheel.

• But pyroma fails on this
• A package with such a license expression requires setuptools>=77 in the build-system, otherwise you get an error when building the sdist or wheel.
• Can you then actually install such source dist or wheel with an older setuptools? Yes, that part at least seems to work. In a virtualenv with an older setuptools, I can call bin/pip install --no-build-isolation and still install a source dist or wheel that has an SPDX license expression.
• I just realised: the SPDX license expressions are only a thing in pyproject.toml. If all your package data is still defined in setup.py, you can put whatever you want in the license keyword.

Some links:

• https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license
• https://peps.python.org/pep-0639/
• https://setuptools.pypa.io/en/latest/userguide/pyproject_config.html