Ensure _apply_pyproject sets field on dist.metadata object not on dist

[abravalheri]

Summary of changes

Not sure why the tests where not failing, but I think this is the way we ensure dist.metadata.license_expression gets set instead of dist.license_expression.

(I guess the test hasattr(dist.metadata, field) is true? Is it always true? If not when it is false?)

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
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[abravalheri]

/cc @cdce8p

[cdce8p]

(I guess the test hasattr(dist.metadata, field) is true? Is it always true? If not when it is false?)

I did add license_expression to _DISTUTILS_UNSUPPORTED_METADATA so it's always initialized to None on dist.metadata anyway.

setuptools/setuptools/dist.py

Lines 288 to 292 in 0a4eb8b

	_DISTUTILS_UNSUPPORTED_METADATA = {
	'long_description_content_type': lambda: None,
	'project_urls': dict,
	'provides_extras': dict, # behaves like an ordered set
	'license_expression': lambda: None,

[abravalheri]

Thank you I will add it just in case.