[tooling] Update license definition in project file

[the-glu]

Follow #20 and #18 (to drop old python support)

Update the license definition in the project file.

Remove the warning that was shown in builds:

 !!

        ********************************************************************************
        Please use a simple string containing a SPDX expression for `project.license`. You can also use `project.license-files`. (Both options available on setuptools>=77.0.0).

        By 2026-Feb-18, you need to update your project and remove deprecated calls
        or your builds will no longer be supported.

        See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
        ********************************************************************************

!!

[mickmis]

Reviewed b99894c
LGTM