Become PEP 639 compliant

[MicahGale]

PEP 639 changes the metadata around license to be expression based.

With PEP 639 flowing down to pyproject.toml we should go from:

license = {file="LICENSE"}

to:

license = "MIT"
license-files= ["LICENSE"]

In my experimenting setuptools (up to version 75.8.0) does not like this and thinks its invalid:

Traceback (most recent call last):
  File "/home/mgale/mambaforge/lib/python3.12/site-packages/pyproject_hooks/_in_process/_in_process.py", line 389, in <module>
    main()
  File "/home/mgale/mambaforge/lib/python3.12/site-packages/pyproject_hooks/_in_process/_in_process.py", line 373, in main
    json_out["return_val"] = hook(**hook_input["kwargs"])
                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/mgale/mambaforge/lib/python3.12/site-packages/pyproject_hooks/_in_process/_in_process.py", line 317, in get_requires_for_build_sdist
    return hook(config_settings)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/build-env-u3mq52vy/lib/python3.12/site-packages/setuptools/build_meta.py", line 337, in get_requires_for_build_sdist
    return self._get_build_requires(config_settings, requirements=[])
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/build-env-u3mq52vy/lib/python3.12/site-packages/setuptools/build_meta.py", line 304, in _get_build_requires
    self.run_setup()
  File "/tmp/build-env-u3mq52vy/lib/python3.12/site-packages/setuptools/build_meta.py", line 320, in run_setup
    exec(code, locals())
  File "<string>", line 1, in <module>
  File "/tmp/build-env-u3mq52vy/lib/python3.12/site-packages/setuptools/__init__.py", line 117, in setup
    return distutils.core.setup(**attrs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/build-env-u3mq52vy/lib/python3.12/site-packages/setuptools/_distutils/core.py", line 160, in setup
    dist.parse_config_files()
  File "/tmp/build-env-u3mq52vy/lib/python3.12/site-packages/setuptools/dist.py", line 652, in parse_config_files
    pyprojecttoml.apply_configuration(self, filename, ignore_option_errors)
  File "/tmp/build-env-u3mq52vy/lib/python3.12/site-packages/setuptools/config/pyprojecttoml.py", line 72, in apply_configuration
    config = read_configuration(filepath, True, ignore_option_errors, dist)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/build-env-u3mq52vy/lib/python3.12/site-packages/setuptools/config/pyprojecttoml.py", line 140, in read_configuration
    validate(subset, filepath)
  File "/tmp/build-env-u3mq52vy/lib/python3.12/site-packages/setuptools/config/pyprojecttoml.py", line 61, in validate
    raise ValueError(f"{error}\n{summary}") from None
ValueError: invalid pyproject.toml config: `project.license`.
configuration error: `project.license` must be valid exactly by one definition (2 matches found):

    - keys:
        'file': {type: string}
      required: ['file']
    - keys:
        'text': {type: string}
      required: ['text']


ERROR Backend subprocess exited when trying to invoke get_requires_for_build_sdist

This is probably the fact that setuptools doesn't support PEP 639 yet I think, but could be operator error