MNT: drop deprecated License classifier, update related metadata (PEP 639)

[neutrinoceros]

No description provided.

[neutrinoceros]

technically unrelated to my initial goal here, but I also fixed the following deprecation warning

        ********************************************************************************
        With Python 2.7 end-of-life, support for building universal wheels
        (i.e., wheels that support both Python 2 and Python 3)
        is being obviated.
        Please discontinue using this option, or if you still need it,
        file an issue with pypa/setuptools describing your use case.

        This deprecation is overdue, please update your project and remove deprecated
        calls to avoid build errors in the future.
        ********************************************************************************