Fix deprecated warning when building the package

[loonies]

Description

AUR package maintainer here.

When building the package we get this warning

--- SNIP ---
/usr/lib/python3.13/site-packages/setuptools/config/_apply_pyprojecttoml.py:82: 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.                                                                                         
        ********************************************************************************                                                                                                       
                                                                                                                                                                                               
!!
--- SNIP ---

This PR fixes deprecated license specification format in pyproject.toml.

The change should be backward compatible as per specification:

The legacy deprecated Core Metadata License field, license key table subkeys (text and file) in the pyproject.toml [project] table and license classifiers retain backwards compatibility. A removal is left to a future PEP and a new version of the Core Metadata specification.

https://peps.python.org/pep-0639/#backwards-compatibility

[georgesittas]

Thank you :)

FYI @tobymao about getting this in.