[ussuri][goal] Cleanup for python 2.7 drop

OpenStack is dropping the py2.7 support in ussuri cycle.

puppet-* repos have to cleanup the requirement and tox
for py2.7 drop.

Complete discussion & schedule can be found in
- http://lists.openstack.org/pipermail/openstack-discuss/2019-October/010142.html
- https://etherpad.openstack.org/p/drop-python2-support

Ussuri Communtiy-wide goal:
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: I7ae860b5e53884ff990397c752d466fa450eb7b4

doc/requirements.txt

@@ -1,6 +1,5 @@
 # This is required for the docs build jobs
-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
 openstackdocstheme>=1.20.0 # Apache-2.0
 
 # This is required for the releasenotes build jobs

tox.ini

@@ -1,12 +1,13 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 skipsdist = True
 envlist = releasenotes
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 
 [testenv:releasenotes]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html