CI: run python2-flake8

With lots of warnings/errors disabled for now. This catches basic things at
least like undefined variables, typos etc and should make sure we don't
introduce any syntax errors.
2 jobs for py2-flake8 in 11 minutes and 52 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #427485
test

00:02:24

passed #427486
ubuntu-19.04-glib

00:11:52