Fix license definition in project metadata

[mschoettle]

Summary

The license definition in pyproject.toml is not valid according to the specification.

As a result, the PyPI API returns license: null for this package: https://pypi.org/pypi/attrs/json

This PR fixes this.

Pull Request Check List

• Do not open pull requests from your main branch – use a separate branch!
  • There's a ton of footguns waiting if you don't heed this warning. You can still go back to your project, create a branch from your main branch, push it, and open the pull request from the new branch.
  • This is not a pre-requisite for your your pull request to be accepted, but you have been warned.
• Added tests for changed code.
  Our CI fails if coverage is not 100%.
• New features have been added to our Hypothesis testing strategy.
• Changes or additions to public APIs are reflected in our type stubs (files ending in .pyi).
  • ...and used in the stub test file tests/typing_example.py.
  • If they've been added to attr/__init__.pyi, they've also been re-imported in attrs/__init__.pyi.
• Updated documentation for changed code.
  • New functions/classes have to be added to docs/api.rst by hand.
  • Changes to the signatures of @attr.s() and @attrs.define() have to be added by hand too.
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
    The next version is the second number in the current release + 1.
    The first number represents the current year.
    So if the current version on PyPI is 22.2.0, the next version is gonna be 22.3.0.
    If the next version is the first in the new year, it'll be 23.1.0.
    • If something changed that affects both attrs.define() and attr.s(), you have to add version directives to both.
• Documentation in .rst and .md files is written using semantic newlines.
• Changes (and possible deprecations) have news fragments in changelog.d.
• Consider granting push permissions to the PR branch, so maintainers can fix minor issues themselves without pestering you.

[webknjaz]

Does twine check --strict not catch this? 🤔 Is it not running in CI?

[hynek]

It didn't because it was completely OK, using a widely-supported, almost-done PEP 639 SPDX identifier which has been the default for Hatch projects for years now BTW.

Given PyPI's web interface has always shown the trove ID classifiers for license, I find it extremely unfortunate that the API chose to use a different source and breaking the ecosystem like this.

[webknjaz]

Yeah, the free-form field still shows up additionally. I hope Karolina gets to finish the PEP this year..