"""
def _update_class(oldclass, newclass):
    ...
    for name in oldnames & newnames - {"__dict__", "__doc__"}:
        setattr(oldclass, name,  _update(olddict[name], newdict[name]))
    return oldclass
"""

I assume __doc__ is skipped because a string can't be updated in place.  But since oldclass is returned, __doc__ should still be replaced with the updated documentation.