[stable-only] Cap bandit and fix lower-constraints

The 1.6.3 [1] release has dropped support for py2 [2] but the release
is faulty and pip still picks it up for py2 [3][4], so cap to 1.6.2
when using py2.

Sphinx requirements also fixed to make requirements-check job pass.

[1] https://github.com/PyCQA/bandit/releases/tag/1.6.3
[2] PyCQA/bandit#615
[3] PyCQA/bandit#663
[4] PyCQA/bandit#665

Change-Id: I12478b0b85aad91bcf71500e0cef59698c528417

test-requirements.txt

@@ -10,8 +10,10 @@ stestr>=2.0.0 # Apache-2.0
 
 # These are needed for docs generation
 openstackdocstheme>=1.18.1 # Apache-2.0
-sphinx!=1.6.6,!=1.6.7,>=1.6.2 # BSD
+sphinx!=1.6.6,!=1.6.7,>=1.6.2,<2.0.0;python_version=='2.7'  # BSD
+sphinx!=1.6.6,!=1.6.7,>=1.6.2;python_version>='3.4' # BSD
 reno>=2.5.0 # Apache-2.0
 
 # Bandit security code scanner
-bandit>=1.1.0 # Apache-2.0
+bandit>=1.1.0,<1.6.3;python_version=='2.7' # Apache-2.0
+bandit>=1.1.0;python_version>='3.5' # Apache-2.0