Support for PEP 639

PEP 639 defines a specification how licenses can be added to python projects. Without the correct markers, the license cannot automatically be tested for Open Source compatibility.

Please have a look at #258


**Relevant documentation**
https://peps.python.org/pep-0639/
https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license
https://spdx.org/licenses/