Here it says:

We have changed sort to use mergesort, which for me is rather surprising since I have been told since I was a toddler to use quicksort. However, the old behavior can be controlled using the sort module; we even have a mystery stable quicksort!

So the switch from quicksort to merge sort apparently happened with 5.8.0. Gee .. I'm out of date. Thanks for pointing that out as well.