Update license metadata in pyproject.toml according to PEP 639

[facutuesca]

setuptools now warns when building a project that uses license classifiers:

DEBUG /github/home/.cache/uv/builds-v0/.tmpjaMf3z/lib/python3.13/site-packages/setuptools/dist.py:759: SetuptoolsDeprecationWarning: License classifiers are deprecated.
DEBUG !!
DEBUG 
DEBUG         ********************************************************************************
DEBUG         Please consider removing the following classifiers in favor of a SPDX license expression:
DEBUG 
DEBUG         License :: OSI Approved :: MIT License
DEBUG 
DEBUG         See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
DEBUG         ********************************************************************************
DEBUG 
DEBUG !!

This is due to PEP 639, which changes the way the licenses (and their files) are specified in the project metadata (see also the packaging guide here). This is supported since version 77.0.3 of setuptools.

This PR changes the metadata accordingly, and bumps the minimum version of setuptools to ensure it's supported.

cc @woodruffw @alex

[alex]

I think we want to leave the setuptools version alone (for 3.7 support) and just update the metadata. People on old setuptools will get marginally wrong stuff, and I guess that's ok.

[facutuesca]

ah but I think that it will cause their builds to outright fail:

If you get a build error that license should be a dict/table, your build backend doesn’t yet support the new format. See the above section for more context. The now deprecated format is described in PEP 621.

(from https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license)

[alex]

Actually, why is setuptools even processing this? We use maturin for builidng the package?

[facutuesca]

Actually, why is setuptools even processing this? We use maturin for builidng the package?

it seems to originate from this uv pip install call: https://github.com/pyca/cryptography/actions/runs/14663972603/job/41154408745#step:10:76

[facutuesca]

🤦 the warning is not coming from cryptography, but rather while building cffi: https://github.com/pyca/cryptography/actions/runs/14663972603/job/41154408745#step:10:928

cffi is the package that has that License :: OSI Approved :: MIT License classifier

[alex]

heh. well, probably good to do this cleanup either way, since maturin appears to handle it properly in all versions.