SetuptoolsDeprecationWarning: License classifiers are deprecated

[qarkai]

Got warning during build:

SetuptoolsDeprecationWarning: License classifiers are deprecated.
        ********************************************************************************
        Please consider removing the following classifiers in favor of a SPDX license expression:

        License :: OSI Approved :: GNU General Public License v3 (GPLv3)

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