In a project using

[project]
license = { file = 'LICENSE' }

with the content of LICENSE being

some license

including the trailing newline, with pyproject-metadata 0.8 I obtain a RFC822 serialization of the metadata that looks like:

License: some license\n

with pyproject-metadata 0.9.0b7 I obtain:

License: some license\n
         \n

where I made the newlines explicit in the RFC822 serialization. When the metadata is read back by packaging.metadata.parse_email() the latter results in a license field having content some license\n . We can argue that whitespace at the end of the text field is not significant, but maybe we should keep stripping the trailing newline.