Update licensing info in pyproject.toml

[wkoot]

From build log:

    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.
    ********************************************************************************

And:

    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.
    ********************************************************************************