`project.license` as a TOML table / license classifiers are deprecated

[llucax]

What's needed?

We need to stop using the deprecated license = { text = "MIT" } and the "License :: ..." classifier.

Proposed solution

1. Use license = "MIT" instead, or license = "LicenseRef-Proprietary" for proprietary.
2. Remove "License :: OSI Approved :: MIT License"

• Update cookiecutter template
• Create migration step in the migration script

Use cases

No response

Alternatives and workarounds

No response

Additional context

Errors

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.
        ********************************************************************************

setuptools.errors.InvalidConfigError: License classifiers have been superseded by license expressions (see https://peps.python.org/pep-0639/). Please remove:

License :: OSI Approved :: MIT License

More info

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

[llucax]

This was fixed by #503.