Add license expression data

[TG1999]

Currently we only provide declared_license and not license_expression. We should also provide license_expression data provided from pypi API

[sschuberth]

Release https://github.com/aboutcode-org/python-inspector/releases/tag/v0.13.1 says to fix this, so can we close this?

[TG1999]

@sschuberth yes!