Travis: Add dependency on python3
Travis: Add dependency on python3

The testsuite now requires python >=3.0. See #12909.

olsner/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,olsner/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,mettekou/ghc,olsner/ghc,shlevy/ghc,mettekou/ghc,olsner/ghc,mettekou/ghc,olsner/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,mettekou/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,olsner/ghc,shlevy/ghc,mettekou/ghc,ezyang/ghc,mettekou/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,mettekou/ghc,olsner/ghc,shlevy/ghc