[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: Ie17b61301a941ff554de9fcb8985b5b1e4096113

.zuul.yaml

@@ -1,10 +1,4 @@
 - project:
     templates:
       - openstack-specs-jobs
-      - openstack-python3-train-jobs
-    check:
-      jobs:
-        - openstack-tox-py27
-    gate:
-      jobs:
-        - openstack-tox-py27
+      - openstack-python3-ussuri-jobs

requirements.txt

@@ -2,7 +2,6 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 pbr>=2.0.0
-sphinx>=1.5.1,!=1.6.6,!=1.6.7,<2.0.0;python_version=='2.7'  # BSD
 sphinx>=1.5.1,!=1.6.6,!=1.6.7,!=2.1.0;python_version>='3.4'  # BSD
 sphinxcontrib-blockdiag
 openstackdocstheme>=1.20.0 # Apache-2.0

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-minversion = 1.6
-envlist = docs,py27,py37
+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}
@@ -14,9 +16,7 @@ commands =
     doc8 specs/
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 commands = sphinx-build -W -b html doc/source doc/build/html