This is IMHO broken.

1. _ensure_list() allows strings, because, documentation says, they are split in finalize_options(). But finalize_options() does only split keywords and platforms. It does _not_ split classifiers.

2. there's no need that keywords, platforms and classifiers must be a list. keywords and platforms can be any iterable, and classifiers can be any non text-like iterable. 

Indeed, keywords are written to file using ','.join(), and platforms and classifiers are written using DistributionMetadata._write_list(). They both accepts any iterable, so I do not understand why such a strict requirement.