[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 requirement updated 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: Id412a175c8b2a9c8c62105f5a6205b1330a9883f

test-requirements.txt

@@ -7,10 +7,11 @@ testrepository>=0.0.18 # Apache-2.0/BSD
 testtools>=2.2.0 # MIT
 
 openstackdocstheme>=1.18.1 # Apache-2.0
-sphinx>=1.6.2 # BSD
+sphinx>=1.6.2,!=1.6.6,!=1.6.7,<2.0.0;python_version=='2.7'  # BSD
+sphinx>=1.6.2,!=1.6.6,!=1.6.7;python_version>='3.4'  # BSD
 
 # Bandit security code scanner
-bandit>=1.1.0 # Apache-2.0
+bandit>=1.1.0,<=1.6.2 # Apache-2.0
 
 pymongo!=3.1,>=3.0.2 # Apache-2.0