Update `pyproject.toml` license metadata to fix setuptools deprecations

[Natalia Bidart]

Recent versions of setuptools (≥77.0.0) deprecate using project.license as a TOML table and discourage use of license classifiers.

* Creating isolated environment: venv+pip...
* Installing packages in isolated environment:
  - setuptools>=75.8.1
* Getting build dependencies for sdist...
/tmp/build-env-d9cqd9kh/lib/python3.12/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.
        ********************************************************************************

!!
  corresp(dist, value, root_dir)
/tmp/build-env-d9cqd9kh/lib/python3.12/site-packages/setuptools/config/_apply_pyprojecttoml.py:61: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

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

        License :: OSI Approved :: BSD License

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

!!
  dist._finalize_license_expression()
/tmp/build-env-d9cqd9kh/lib/python3.12/site-packages/setuptools/dist.py:759: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

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

        License :: OSI Approved :: BSD License

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

!!
  self._finalize_license_expression()
running egg_info

[David Smith]

Duplicate of https://code.djangoproject.com/ticket/35961 ?

[Natalia Bidart]

Replying to David Smith:

Duplicate of https://code.djangoproject.com/ticket/35961 ?

Yes, thank you David, I forgot about that one! (I even did a review...)