Modernize license handling to use SPDX expressions

[RensDimmendaal]

nbdev_pypi raises the following warnings:

.../.venv/lib/python3.12/site-packages/setuptools/dist.py:759: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

        ********************************************************************************
        Please consider removing the following classifiers in favor of a SPDX license expression:

        License :: OSI Approved :: Apache Software License

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

!!

This PR:

• Replace deprecated license classifiers with modern SPDX expressions
• Remove 'License :: OSI Approved :: *' classifiers that cause setuptools warnings
• Update license mapping to use SPDX identifiers (Apache-2.0, MIT, etc.)
• Simplify license handling logic since we no longer need tuples

[jph00]

This is much nicer - thank you :)