bandit 1.6.3 supports only python>=3.5 and stein jobs with python 2.7 are broken when bandit 1.6.3 is being installed. bandit <1.6.3 supports python 2.7. We have no constraint for test only libraries so we need to cap it. doc/requirements.txt and lower-constraints.txt also needs to be updated to pass the requirements-check and lower-constarints-job respectively. Change-Id: Ia8c69501a1362cb08344aa50d00ed184e9f3d49fchanges/03/768403/3