[stable-only] Cap bandit to 1.6.2

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: Ib2bad4cbd1ac9a63cd8c8e48f5cd5bf8c5cde595

test-requirements.txt

@@ -15,9 +15,11 @@ oslotest>=3.2.0 # Apache-2.0
 coverage!=4.4,>=4.0 # Apache-2.0
 
 # this is required for the docs build jobs
-sphinx!=1.6.6,>=1.6.2 # BSD
+sphinx>=1.6.2,!=1.6.6,<2.0.0;python_version=='2.7'  # BSD
+sphinx>=1.6.2,!=1.6.6;python_version>='3.4'  # BSD
 openstackdocstheme>=1.18.1 # Apache-2.0
 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