Update python_lint shebang to 'python3'
Update python_lint shebang to 'python3'

With the Python 3 changes to the repo, the linter now needs to be called
with python3 explicitly, so that there are no syntax errors.

atrick/swift,gregomni/swift,JGiola/swift,ahoppen/swift,roambotics/swift,atrick/swift,rudkx/swift,glessard/swift,benlangmuir/swift,benlangmuir/swift,ahoppen/swift,gregomni/swift,roambotics/swift,JGiola/swift,roambotics/swift,rudkx/swift,ahoppen/swift,roambotics/swift,rudkx/swift,gregomni/swift,gregomni/swift,ahoppen/swift,JGiola/swift,benlangmuir/swift,rudkx/swift,gregomni/swift,apple/swift,ahoppen/swift,glessard/swift,atrick/swift,benlangmuir/swift,atrick/swift,rudkx/swift,apple/swift,gregomni/swift,glessard/swift,roambotics/swift,atrick/swift,ahoppen/swift,apple/swift,JGiola/swift,glessard/swift,glessard/swift,glessard/swift,JGiola/swift,roambotics/swift,benlangmuir/swift,JGiola/swift,apple/swift,benlangmuir/swift,apple/swift,apple/swift,rudkx/swift,atrick/swift