Run security linter bandit as part of pep8. Pep8 is the usual linter
target and thus let's use it there instead of starting another node
for this short-running job.
Also reduced the lines of context from 5 to 3 since it shouldn't
matter to most.
Change-Id: I54a4fccebb375517f9cd129f62f8f0c795b6edcc