'license' field in pyproject.toml to be updated

Problem description

The [project.license] field in pyproject.toml is deprecated since version 2.4 (PEP 639) of the core metadata specification and replaced by license-expression.

Solution

Instead a [License-expression] should be used.

Todo

Replace the [project.license] field (which refers only to AGPL-3.0-or-later) with a [project.license-expression] field that covers all three licenses that are currently used.

Open questions

How should the license-expression be formulated? I'm not sure weather to concatenate our licenses with AND or OR statements:

• OR expressions to state that each file has only one of the licenses
• AND expressions to state that the whole project contains files with those licenses

so weather "AGPL-3.0-or-later OR CC-BY-SA-4.0 OR CC0-1.0" or "AGPL-3.0-or-later AND CC-BY-SA-4.0 AND CC0-1.0"?