Add license information to pyproject.toml files for multiple libraries

[Chris Mullins (cleemullins)]

Fixes #160

According to PEP 639 (Improving License Clarity with Better Package Metadata):

The old License :: OSI Approved :: MIT License classifier is deprecated
Modern packages should use the license field with SPDX identifiers
This provides better machine-readable license information
PyPI and packaging tools now prefer this format

As described in #160, this directly addresses the build warnings that look like:

/tmp/build-env-cdo14fs1/lib/python3.11/site-packages/setuptools/config/_apply_pyprojecttoml.py:61: 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.
    ********************************************************************************

!!