test buster instead of stretch

Signed-off-by: Mattia Rizzolo <mattia@debian.org>
3 jobs for live in 5 minutes and 6 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #162084
buster-bpo

00:05:06

85.0%
passed #162085
flake8

00:01:56

passed #162083
unstable

00:05:04

85.0%