ci: install six for testing

6 jobs for ci in 3 minutes and 27 seconds
latest
Status Job ID Name Coverage
  Build
passed #118747
build:debian:jessie

00:03:10

passed #118748
build:debian:stretch

00:01:07

passed #118746
build:el7

00:00:50

passed #118745
build:tarball

00:00:18

 
  Test
passed #118749
test:python2.7

00:00:14

passed #118750
test:python3.6

00:00:16