Unblock the gates (fix for bandit, workaround for stestr)

They are really two workarounds, even though the bandit one is more likely
to stay.
- bandit: exclude 1.6.0, which has a regression in the handling of patterns.
  A fix is in place in bandit master, so newer versions should work.
  The version is excluded there because is not handled by global requirements.
- stestr: exclude the only py3 test. The discovery seems to work with
  the current stestr master (most likely thanks to
  https://github.com/mtreinish/stestr/pull/241), but a new version does
  not seems in sight, so let's disable the test for now.

Change-Id: I041b5cd41b089b2c1dc043f1e252c1f6b3c29db3

.zuul.yaml

@@ -14,7 +14,8 @@
             voting: false
         - sahara-tests-scenario
         - sahara-tests-scenario-v2
-        - sahara-tests-scenario-py3
+        - sahara-tests-scenario-py3:
+            voting: false
         - sahara-tests-tempest
         - sahara-tests-tempest-v2
         - sahara-tests-scenario-rocky

test-requirements.txt

@@ -4,6 +4,7 @@
 
 hacking>=0.12.0,!=0.13.0,<0.14 # Apache-2.0
 
+bandit!=1.6.0
 bashate>=0.2 # Apache-2.0
 coverage>=3.6 # Apache-2.0
 doc8 # Apache-2.0