A lot of projects write in that style e.g. compatible with both python2 and python3, it's really common because there's so much py2 deployed (was default on centOS until very recently, still default on osx, etc.)

Nearly every py3 feature was backported to 2 you just need to write it in a compatible way. I'm seeing some drop py2 support now though. Which I'm fine with, I haven't written python2 code in maybe 6 or 7 years now.