[ussuri][goal] Drop python 2.7 support

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

Fix test_titles.py for changed generator syntax.

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

Change-Id: I334a97d3a0d144b233f51dc864eb56eb6057a064

.zuul.yaml

@@ -3,7 +3,7 @@
       - openstack-specs-jobs
     check:
       jobs:
-        - openstack-tox-py27
+        - openstack-tox-py36
     gate:
       jobs:
-        - openstack-tox-py27
+        - openstack-tox-py36

requirements.txt

@@ -1,5 +1,4 @@
 openstackdocstheme>=1.11.0
-sphinx!=1.6.6,!=1.6.7,<2.0.0;python_version=='2.7'  # BSD
 sphinx!=1.6.6,!=1.6.7;python_version>='3.4'  # BSD
 testrepository>=0.0.18
 testtools>=0.9.34

tests/test_titles.py

@@ -111,7 +111,7 @@ class TestTitles(testtools.TestCase):
         # Get the current release directory - since we're moving up
         # alphabetically, grab the highest name in the spec directory
         generator = os.walk('specs')
-        dirname, subdirs, files = generator.next()
+        dirname, subdirs, files = next(generator)
         current_release = max(subdirs)
         found = False
         for dirname, subdirs, files in generator:

tox.ini

@@ -1,9 +1,10 @@
 [tox]
 minversion = 1.6
-envlist = docs,py27
+envlist = docs,py3
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
@@ -11,11 +12,9 @@ deps = -r{toxinidir}/requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:spelling]