Bump bandit version
Bump bandit version

This patch bumps bandit allowed version to >=1.6.0,<1.7.0 in order to
avoid the errors detailed here https://github.com/PyCQA/bandit/pull/393

Change-Id: I23fd4082e2566242b5fce093a4685c97663c947f
Signed-off-by: Moisés Guimarães de Medeiros <38931a700a64030c6e1883420cbba039aadcb37a@redhat.com>

openstack/oslo.privsep