Add setuptools to pyproject.toml

[henryiii]

This adds setuptools' schema to pyproject.toml, using a modified form from validate-pyproject used in setuptools itself.

This exposed a bug in the current test suite - tool.setuptools.packages.find.where must be an array. If you try to run it with the file (src/test/pyproject/simple.toml) as it exists in the current examples:

ValueError: invalid pyproject.toml config: `tool.setuptools.packages`.
configuration error: `tool.setuptools.packages` must be valid exactly by one definition (0 matches found):

    - type: array
      items:
        type: string
        at least one of the following:
          - {format: 'python-module-name'}
          - {format: 'pep561-stub-name'}
    - type: table
      additional keys: False
      keys:
        'find':
          type: table
          additional keys: False
          keys:
            'where':
              type: array
              items: {type: string}
            'exclude':
              type: array
              items: {type: string}
            'include':
              type: array
              items: {type: string}
            'namespaces': {type: boolean}


ERROR Backend subprocess exited when trying to invoke get_requires_for_build_sdist

[henryiii]

This was a bug! Setuptools requires an array here.

[henryiii]

I discovered the "unknownFormats" field, which allows this to be closer to the upstream version.

[madskristensen]

Thanks