[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: Icd02e70d4884c252ed4ada7cd224c0451854cf31

.zuul.yaml

@@ -3,7 +3,7 @@
       - openstack-specs-jobs
     check:
       jobs:
-        - openstack-tox-py27
+        - openstack-tox-py37
     gate:
       jobs:
-        - openstack-tox-py27
+        - openstack-tox-py37

requirements.txt

@@ -1,6 +1,5 @@
 openstackdocstheme>=1.18.1 # Apache-2.0
 pbr>=0.6,!=0.7,<1.0
-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
 stestr>=2.0.0
 testtools>=0.9.34

tox.ini

@@ -1,11 +1,13 @@
 [tox]
 # Hold back to 1.4, since that's what's in Fedora 20 repos
 # and we don't need anything newer for nova-specs tests
-minversion = 1.4
-envlist = docs,py27
+minversion = 3.1.1
+envlist = docs,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
 install_command = pip install -U {opts} {packages}
@@ -13,9 +15,7 @@ deps = -r{toxinidir}/requirements.txt
 commands = stestr run --slowest {posargs}
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 commands = python setup.py build_sphinx