add lower-constraints tox environment and job

Update the minimum for PyYAML to a version that appears on PyPI.

Change-Id: Icd8dbdeb4dcbf45b5e0d5efb5e66baf3ef9749f5
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

.zuul.yaml

@@ -3,3 +3,4 @@
       - openstack-python-jobs
       - openstack-python35-jobs
       - publish-openstack-docs-pti
+      - openstack-lower-constraints-jobs

lower-constraints.txt

@@ -0,0 +1,5 @@
+Sphinx==1.5.1
+docutils==0.11
+PyYAML==3.10.0
+six==1.9.0
+dulwich==0.15.0

requirements.txt

@@ -3,6 +3,6 @@
 # process, which may cause wedges in the gate later.
 
 pbr
-PyYAML>=3.1.0
+PyYAML>=3.10
 six>=1.9.0
 dulwich>=0.15.0  # Apache-2.0

tox.ini

@@ -20,6 +20,12 @@ commands =
   flake8
   reno -q lint
 
+[testenv:lower-constraints]
+basepython = python3
+deps =
+  -c{toxinidir}/lower-constraints.txt
+  {[testenv]deps}
+
 [testenv:venv]
 basepython = python3
 commands = {posargs}