test: make `gen-output-file-map` Python 3 friendly
test: make `gen-output-file-map` Python 3 friendly

Adjust `gen-output-file-map.py` to be friendly to Python 3 by using
`io.open` instead of `open`.  This allow the use of `encoding` and
`newline` named parameters to write out files using UTF-8 encoded, UNIX
newline delimited files.

atrick/swift,jckarter/swift,benlangmuir/swift,airspeedswift/swift,apple/swift,roambotics/swift,gregomni/swift,airspeedswift/swift,airspeedswift/swift,xwu/swift,hooman/swift,atrick/swift,xwu/swift,nathawes/swift,CodaFi/swift,hooman/swift,rudkx/swift,atrick/swift,ahoppen/swift,CodaFi/swift,gregomni/swift,CodaFi/swift,parkera/swift,nathawes/swift,jckarter/swift,roambotics/swift,jmgc/swift,airspeedswift/swift,parkera/swift,tkremenek/swift,allevato/swift,jmgc/swift,xwu/swift,tkremenek/swift,parkera/swift,roambotics/swift,hooman/swift,airspeedswift/swift,benlangmuir/swift,benlangmuir/swift,nathawes/swift,ahoppen/swift,tkremenek/swift,JGiola/swift,hooman/swift,ahoppen/swift,JGiola/swift,jckarter/swift,glessard/swift,glessard/swift,CodaFi/swift,atrick/swift,nathawes/swift,atrick/swift,apple/swift,jmgc/swift,allevato/swift,jckarter/swift,JGiola/swift,allevato/swift,CodaFi/swift,glessard/swift,allevato/swift,apple/swift,jckarter/swift,parkera/swift,glessard/swift,parkera/swift,JGiola/swift,xwu/swift,allevato/swift,airspeedswift/swift,CodaFi/swift,hooman/swift,gregomni/swift,parkera/swift,benlangmuir/swift,benlangmuir/swift,airspeedswift/swift,xwu/swift,jmgc/swift,allevato/swift,nathawes/swift,jmgc/swift,xwu/swift,rudkx/swift,gregomni/swift,tkremenek/swift,parkera/swift,nathawes/swift,gregomni/swift,nathawes/swift,ahoppen/swift,rudkx/swift,allevato/swift,roambotics/swift,benlangmuir/swift,gregomni/swift,rudkx/swift,parkera/swift,jmgc/swift,CodaFi/swift,jckarter/swift,jckarter/swift,apple/swift,hooman/swift,JGiola/swift,ahoppen/swift,tkremenek/swift,ahoppen/swift,apple/swift,rudkx/swift,hooman/swift,roambotics/swift,glessard/swift,jmgc/swift,glessard/swift,atrick/swift,xwu/swift,rudkx/swift,tkremenek/swift,apple/swift,roambotics/swift,tkremenek/swift,JGiola/swift