Build pdf doc

The is one of community goals that each project could produce a
single PDF file. The pdf should be in the output of openstack-tox-docs
job.

TeX packages are required to build PDF locally, following is recommended:

* inkscape
* texlive-latex-base
* texlive-latex-extra
* texlive-fonts-recommended

More about the goal:
https://governance.openstack.org/tc/goals/train/pdf-doc-generation.html
https://etherpad.openstack.org/p/train-pdf-support-goal
https://etherpad.openstack.org/p/pdf-goal-train-common-problems

Change-Id: I8ebbe9ea1bb2914061a8307f3edfabab166a3c38
diff --git a/test-requirements.txt b/test-requirements.txt
index b5667d2..fb97fc6 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -6,6 +6,6 @@
 
 sphinx!=1.6.6,!=1.6.7,>=1.6.2,<2.0.0;python_version=='2.7' # BSD
 sphinx!=1.6.6,!=1.6.7,>=1.6.2;python_version>='3.4' # BSD
-openstackdocstheme>=1.18.1 # Apache-2.0
+openstackdocstheme>=1.20.0 # Apache-2.0
 
 reno>=2.5.0 # Apache-2.0