Cleanup py27 support

This repo is now testing only with Python 3, so let's make
a few cleanups:
- Update requirements, no need for python_version anymore
- Use newer openstackdocstheme version
- Remove install_command from tox.ini, the default is fine

Change-Id: Ib57fed37abe408b047b1ec364aea945ad06d75a5

doc/requirements.txt

@@ -2,7 +2,6 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 
-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,!=2.1.0,>=1.6.2;python_version>='3.4'  # BSD
+sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2  # BSD
 sphinx-testing # BSD
-openstackdocstheme>=1.19.0 # Apache-2.0
+openstackdocstheme>=1.31.2 # Apache-2.0

tox.ini

@@ -14,8 +14,6 @@ whitelist_externals =
   mkdir
   rm
   sed
-install_command =
-  pip install -U --force-reinstall {opts} {packages}
 setenv =
   VIRTUAL_ENV={envdir}
   OS_TEST_PATH=./training-labs/tests/