ci: test python 3.9 too

5 jobs for master in 31 seconds (queued for 3 seconds)
latest
Status Name Job ID Coverage
  Test
passed pytest_3.6 #1231951

00:00:31

passed pytest_3.7 #1231950

00:00:30

passed pytest_3.8 #1231949

00:00:28

passed pytest_3.9 #1231948

00:00:30

passed pytest_latest #1231947

00:00:28