Update license format in pyproject.toml

[ptgrogan]

SetuptoolsDeprecationWarning: project.license as a TOML table is deprecated
!!

    ********************************************************************************
    Please use a simple string containing a SPDX expression for `project.license`. You can also use `project.license-files`. (Both options available on setuptools>=77.0.0).

    By 2026-Feb-18, you need to update your project and remove deprecated calls
    or your builds will no longer be supported.

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

!!

[ptgrogan]

Setuptools >= 75.4.0 requires Python >= 3.9 which cannot be accommodated in TAT-C as of this time.

[ptgrogan]

This issue has been resolved in #141 by dropping support for Python < 3.10.