setup.py: License SPDX

[ax3l]

The license metadata was replaced with a standard SPDX identifier.

Warning:

setuptools\dist.py:765: SetuptoolsDeprecationWarning: License classifiers are deprecated.

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Please consider removing the following classifiers in favor of a SPDX license expression:

License :: OSI Approved :: GNU Lesser General Public License v3 or later (LGPLv3+)

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

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