Add lower-constraints job to tox

We want to be running this on any new repos we create.

Change-Id: Ic3c048a2ea2e289085e40993927bbb25829804c3

oslo.{{cookiecutter.module_name}}/lower-constraints.txt

@@ -0,0 +1,7 @@
+# The order of packages is significant, because pip processes them in the order
+# of appearance. Changing the order has an impact on the overall integration
+# process, which may cause wedges in the gate later.
+
+Babel==1.3.0
+hacking==0.10.0
+oslotest==1.5.1

oslo.{{cookiecutter.module_name}}/tox.ini

@@ -12,6 +12,12 @@ deps =
     -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}
 
+[testenv:lower-constraints]
+basepython = python3
+deps = -c{toxinidir}/lower-constraints.txt
+       -r{toxinidir}/requirements.txt
+       -r{toxinidir}/test-requirements.txt
+
 [testenv:pep8]
 commands = flake8 {posargs}