Python 2.7 is currently supported. It's not a great idea, but at least defensible, to still be developing against it today. But the moment it becomes officially obsolete and shops have to justify paying special support contracts to maintain it, I think it'll become very hard to explain why that's a good business decision.

There are valid reasons for using 2.7 today. I think it's professional negligence to not at least be planning for the inevitable upgrade to the current version. Honestly, it's almost never as hard as you'd think. The most common hurdles are str/bytes incompatibility, and 3.x only reveals those problems where they already exist in 2.x (where the code is quite possibly not working as expected but silently passing wrong data without warning you).