Hmm... you really use the system Python installation for development? Python is not pathologically backwards compatible, what happens when the next OS version use a newer Python 2 version?
Say, you use containers and so your application is isolated from the host system. When the next version of your host system comes with a new version of Docker, you have to do similar testing and validation anyway.
If I was writing software that had to be used in as many places as possible or sold, I'd absolutely strongly consider using 2.7.