Use SPDX license identifier for metadata

## FEATURE REQUEST [PEP 639](https://peps.python.org/pep-0639/) recommends the use of SPDX license identifiers. I believe it would make sense to adopt them. The required change is minimal. https://spdx.org/licenses/ZPL-2.1.html https://github.com/zopefoundation/RestrictedPython/blob/642202897b419a74b1cb1756315936b45fb75e78/setup.py#L30 ```diff - license='ZPL 2.1', + license='ZPL-2.1', ```