Enable Python 2.7 unit tests

Add a testenv:py27 environment that overrides basepython to 2.7

Unfortunately implicit namespace packages are a Python3 thing [1] so
we have to scatter a few __init__.py's around for the test loader
under python2 to be able to find the unit test directories.

Update documenation to mention this

Needed-By: https://review.openstack.org/592768

[1] https://www.python.org/dev/peps/pep-0420/

Change-Id: I9a653666e8a083fb7f3fbb92589fe0467a41e6e6

doc/source/deprecation.rst

@@ -32,6 +32,18 @@ Before deprecating a feature in Zuul which is used by `zuul-jobs`, the
 usage of the feature must be removed from `zuul-jobs` according to the
 deprecation policy described above.
 
+Python Version Policy
+=====================
+
+``zuul-jobs`` targets Python 2.7 onwards and Python 3.5 onwards (note
+this differs slightly from Ansible upstream, where the policy is 2.6
+onwards unless libraries depend on newer features.  `zuul-jobs` does
+not support Python 2.6).
+
+Library code should be written to be compatible with both.  There are
+some tips on this in `Ansible and Python 3
+<https://docs.ansible.com/ansible/2.5/dev_guide/developing_python_3.html>`__.
+
 .. _zuul-announce: http://lists.zuul-ci.org/cgi-bin/mailman/listinfo/zuul-announce
 .. _zuul-discuss: http://lists.zuul-ci.org/cgi-bin/mailman/listinfo/zuul-discuss
 

tox.ini

@@ -10,6 +10,9 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
            stestr slowest
 
+[testenv:py27]
+basepython = python2.7
+
 [testenv:bindep]
 # Do not install any requirements. We want this to be fast and work even if
 # system dependencies are missing, since it's used to tell you what system