After seeing the months- to years-long process that enterprises have undertaken when migrating from one JDK version to another because of these "obscure areas", I have little empathy for anyone delaying a python upgrade they've been given a 10+ year window for. Ruby 1.8 to 1.9 was similarly painful to python.
The incompatible areas that you explain amount to bug fixes and obscure areas of the language. They aren't anything like Python 2 vs 3.