I just found bug #22232 myself but thanks for pointing it out.

> changing the behavior unconditionally is not an option

At this point, I disagree.  If I do a search on the web, lots of pages referring to str.splitlines() seem it imply that is splits only on \r and \n.  For Python 2 that was correct.  I think most people would be surprised by the Python 3 behaviour.

I looked through the Python stdlib and marked any place str.splitlines() was used.  I have more research to do yet but I think nearly all of these cases will work better (or perhaps correctly) if str.splitlines is changed.