> I would prefer the isinstance(cls, ABCMeta) check to be inside that helper

I had a little debate about this in my mind, I'll change it.

> it's totally clear to me what @total_ordering should do -- it should define __le__, __gt__ and __ge__ in terms of __lt__, and leave __lt__ alone, for some subclass to implement.

Implementing this logic would require more than two lines. I will add it to the PR.