Improve dual-licensing detection

[joshfriend]

Ran into an issue with python-dateutil where the license="Dual License" in setup.cfg, but the trove classifiers report Apache 2.0 and BSD 3-clause. My license check failed because "Dual" was not an approved license.

My suggestion here would be to ignore the plain License: * metadata if Classifier: License :: * metadata exists, instead of combining them into a list:

# Current:
licenses = get_license(dist) + get_license_OSI_classifiers(dist)

# Suggestion
licenses = get_license_OSI_classifiers(dist) or get_license(dist) or ['Unknown']

simplejson is another dual-license example, though their metadata specifies it a bit differently:

License: MIT License
License :: OSI Approved :: MIT License
License :: OSI Approved :: Academic Free License (AFL)