[ussuri][goal] Updates for python 2.7 drop

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

openstack-ansible repos need few updates on requirements
and tox file.

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: Ic6cb1eb547efa7cda472950a49dd8adbd7760ffe

requirements.txt

@@ -3,7 +3,6 @@
 # process, which may cause wedges in the gate later.
 
 # 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,>=1.6.2;python_version>='3.4'  # BSD
+sphinx!=1.6.6,!=1.6.7,>=1.6.2 # BSD
 openstackdocstheme>=1.20.0 # Apache-2.0
 doc8>=0.6.0 # Apache-2.0

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 skipsdist = True
 envlist = docs
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command =
     pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
@@ -27,7 +29,6 @@ setenv =
     VIRTUAL_ENV={envdir}
 
 [testenv:docs]
-basepython = python3
 commands=
     bash -c "rm -rf doc/build"
     doc8 doc
@@ -39,6 +40,5 @@ extensions = .rst
 
 # environment used by the -infra templated docs job
 [testenv:venv]
-basepython = python3
 commands =
     {posargs}