Get a merged dictionary from `cls` bases attribute `attr_name`. MRO defines importance (closest = strongest).

def _dct_from_mro(cls: type, attr_name: str) -> dict:
    """"Get a merged dictionary from `cls` bases attribute `attr_name`. MRO defines importance (closest = strongest)."""
    d = {}
    for c in reversed(cls.mro()):
        d.update(getattr(c, attr_name, {}))
    return d