Add support for modern license entries in `pyproject.toml`

[set-soft]

Describe the solution you'd like
The supported mechanism:

license = { file = "LICENSE" }

Is currently deprecated in favor of SPDX tags, but this tool is forcing the deprecated mechanism, which is confusing for Python developers

[bigcat88]

Is this is about supporting PEP 639 records, like this one:

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

[set-soft]

Yes, in particular SPDX, which is the recommended mechanism.
If I use the file mechanism in a regular Python package, I get a deprecation warning asking to use it.