test: migrate tests to Python3 by default
test: migrate tests to Python3 by default

This changes the python interpreter used to invoke the tests to Python3.
Python2 has been EOL'ed by the Python Software Foundation.  This
migration enables the Swift test suite to run with Python 3 instead.

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