I think this goes too far because it's not realistic to expect type checkers to support arbitrary base classes on NamedTuple. Also, there haven't been any user requests that I can see for multiple inheritance with anything other than Generic. So I'd prefer to merge the other PR that allows multiple inheritance with Generic only.

I also think it makes sense to only allow multiple inheritance with Generic, for now. It's a smaller change; and, there should be opportunity to rethink it in the future if it turns out that people really need arbitrary multiple inheritance for whatever reason.