MAINT: use a PEP 639 project.license SPDX expression

[jorenham]

This resolves the following Python 3..13 build-time 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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See also

• https://peps.python.org/pep-0639/
• https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license

[jorenham]

CI failure seens unrelated, see #135

[Kai-Striega]

Looks good to me!

Can confirm that the CI changes are unrelated. I'd like to fix those first so that we don't have the same failing test on main. I'll (hopefully) merge this over the weekend.

Thanks for your work

[Kai-Striega]

Merging as the underlying test failure has been fixed and the change is appropriate. Thanks @jorenham!