[ussuri][goal] Drop python 2.7 support

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

specs repo either has py27 job or requirement or tox env.

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

Change-Id: I526e776be61e1fc7d3dda83b0cfa936b5306f04b

requirements.txt

@@ -3,7 +3,6 @@
 # process, which may cause wedges in the gate later.
 
 pbr>=2.0.0,!=2.1.0 # Apache-2.0
-sphinx>=1.8.0,<2.0.0;python_version=='2.7' # BSD
 sphinx>=1.8.0,!=2.1.0;python_version>='3.4' # BSD
 testtools>=1.4.0
 yasfb>=0.8.0

tox.ini

@@ -1,7 +1,8 @@
 [tox]
-minversion = 2.0
+minversion = 3.1.1
 envlist = docs,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
 basepython=python3
@@ -16,11 +17,9 @@ commands =
   stestr run --slowest {posargs}
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
@@ -30,7 +29,6 @@ commands =
   python setup.py build_sphinx
 
 [testenv:pdf-docs]
-basepython = python3
 envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 whitelist_externals =
@@ -42,7 +40,6 @@ commands =
   make -C doc/build/pdf
 
 [testenv:pep8]
-basepython = python3
 deps =
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt