SPDX licenses

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

Start by locating the Plone package pyproject.toml files that define license metadata, then read the Packaging guide, PEP 639, and the setuptools configuration notes linked in the issue. Check source distribution and wheel builds with setuptools 77 or newer, installation with older setuptools, and the reported pyroma failure. Done means the affected packages consistently use SPDX expressions without breaking these checks.