New code has no problem explicitly requesting python3, old code needs changing to request Python 2, but if you touch it anyways it's of course safer to make that change.
In case anyone thinks Arch is being crazy here and ignored the PEP (and doesn't read the PEP), note the change here was written in 2011 and approved in 2012 based on Arch's experience moving to python 3 by default in 2010.
New code has no problem explicitly requesting python3, old code needs changing to request Python 2, but if you touch it anyways it's of course safer to make that change.