Update license information in pyproject.toml to use new format

[rwedge]

In a recent run of the release workflow, there were a series of warnings about an upcoming deprecation for how setuptools expects license information to be stored in the pyproject.toml file. We need to update our pyproject.toml by February 18 or the build will start failing.

Relevant warnings:

/tmp/build-env-0pjklcct/lib/python3.13/site-packages/setuptools/config/_apply_pyprojecttoml.py:82: 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.
        ********************************************************************************

!!
  corresp(dist, value, root_dir)
/tmp/build-env-0pjklcct/lib/python3.13/site-packages/setuptools/config/_apply_pyprojecttoml.py:55: SetuptoolsDeprecationWarning: 'tool.setuptools.license-files' is deprecated in favor of 'project.license-files' (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-files for details.
        ********************************************************************************

!!
  _apply_tool_table(dist, config, filename)
/tmp/build-env-0pjklcct/lib/python3.13/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 :: Free for non-commercial use

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