Setuptools license deprecation

[benjeffery]

Getting this on the current build:

/home/benj/projects/tskit/env/lib/python3.12/site-packages/setuptools/dist.py:760: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

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

        License :: OSI Approved :: MIT License

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

[benjeffery]

Shelved for now as support for the new way of doing this is not yet on the conda version of setuptools.