[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: Iac7477a9cfce0458b55d40d5219a315c47abee27

bindep.txt

@@ -0,0 +1,6 @@
+# This is a cross-platform list tracking distribution packages needed for install and tests;
+# see https://docs.openstack.org/infra/bindep/ for additional information.
+
+# graphviz is necessary for documentation build
+graphviz
+

requirements.txt

@@ -3,7 +3,7 @@
 # process, which may cause wedges in the gate later.
 
 pbr!=2.1.0  # Apache-2.0
-sphinx!=1.6.6,!=1.6.7  # BSD
+sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2;python_version>='3.4'  # BSD
 graphviz>=0.4,!=0.5.0  # MIT License
 stestr>=2.0.0
 testtools>=0.9.34

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-minversion = 2.0
-envlist = docs,py27
+minversion = 3.1.1
+envlist = docs,py37
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
 install_command = pip install -U {opts} {packages}
@@ -11,23 +13,19 @@ deps = -r{toxinidir}/requirements.txt
 commands = stestr run --slowest {posargs}
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
     -r{toxinidir}/requirements.txt
 commands = sphinx-build -W -E -b html -d doc/build/doctrees doc/source doc/build/html
 
 [testenv:doc8]
-basepython = python3
 deps = -r{toxinidir}/requirements.txt doc8
 commands = doc8 doc/source
 
 [testenv:autobuild]
-basepython = python3
 whitelist_externals =
     sphinx-autobuild
 commands =