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: If1e25c02adf0fb8f96f7bfb9134c5d170fab15eb
Signed-off-by: Moisés Guimarães de Medeiros <moguimar@redhat.com>

test-requirements.txt

@@ -13,6 +13,6 @@ stestr>=2.0.0 # Apache-2.0
 #  [testenv:cover]
 #  deps = {[testenv]deps} coverage
 coverage!=4.4,>=4.0 # Apache-2.0
-bandit>=1.1.0 # Apache-2.0
+bandit>=1.6.0,<1.7.0 # Apache-2.0
 ddt>=1.0.1 # MIT
 oslo.context>=2.19.2 # Apache-2.0