Remove deprecated classifiers, specify gplv2+

[stephen-huan]

This PR

Remove deprecated classifiers, specify gplv2+

After PEP 639 the license field in pyproject.toml is no longer a table but a SPDX Expression containing a License Identifier. This fixes the following warning.

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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.
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In addition, be more consistent about the license being GPL v2.0 or later (as opposed to v2 only).

• I've read the DCO.
• I've read the Coding Guidelines
• The relevant informations about the changes stands in the commit message, not here in the message of the pull request.
• Code changes follow the style of the files they change.
• Code is tested (provide details).

References

Additional information

• https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license-and-license-files
• https://peps.python.org/pep-0639/
• https://spdx.org/licenses/GPL-2.0-or-later.html