Update pyproject licence metadata

[nhairs]

Noticed this when building 4.0.0.dev0

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.

[V1rg1lee]

Hello, I allowed myself a pull request without prior discussion, since it's a very small change, if you have any questions don't hesitate, I've documented in the PR the reasons for the change.

[nhairs]

So it looks like this is not backwards compatible with Python 3.8 (because of setuptools dropping support for it).

I don't know if there is a build backend that we could move to that would cover all our current versions.

This raises the question of if we drop Python 3.8 support just to support this change (or if we want to have a branch just for 3.8 support).

edit: I'll note that Python 3.8 still makes up >5% of our total downloads so by our own Security Policy and Development Policy we MUST provide security support and SHOULD provide general support (which might be done through a branch).