Make mypy pass

[block-chaynes]

Changes needed in order for mypy to pass

This also fixes a few bugs and some incorrect type hints.

Bugs:

• open_rarity/models/token_metadata.py was using datetime.fromtimestamp but should have been datetime.datetime.fromtimestamp
• RankResolver.get_all_ranks had a definition that didn't match the child implementations
• in open_rarity/resolver/testset_resolver.py the method resolve_collection_data was returning a dict rather than a list as the types and docs implied

Bug left unresolved:

• in open_rarity/resolver/testset_resolver.py the extract_rank method assumes the token_identifier has a token_id field, but this is not true for SolanaMintAddressTokenIdentifier. I'm ignoring the type error for now

[vickygos]

Small comments but lgtm. Thanks for doing this!