Enable mypy 'check_untyped_defs'

[whyscream]

This uncovers various issues with the implementation of get_resource_class(). Now we have mypy passing and the code should work in theory, but the only place where this method is used (in ObjectList), already shows clearly that the current approach won't work.

We need to rethink the way ObjectList works, and hopefully get rid of get_resource_class() entirely.