ci: Travis: test with Python 3.8-dev
ci: Travis: test with Python 3.8-dev

hugovk/coveragepy,nedbat/coveragepy,hugovk/coveragepy,nedbat/coveragepy,nedbat/coveragepy,nedbat/coveragepy,nedbat/coveragepy,hugovk/coveragepy,hugovk/coveragepy,hugovk/coveragepy