[ussuri][goal] Cleanup for python 2.7 drop

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

puppet-* repos have to cleanup the requirement and tox
for py2.7 drop.

Complete discussion & schedule can be found in
- http://lists.openstack.org/pipermail/openstack-discuss/2019-October/010142.html
- https://etherpad.openstack.org/p/drop-python2-support

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

Change-Id: I799a5fee65df662dd393663b25eeda14ac03d6e8

.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 @@
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
-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,!=2.1.0,>=1.6.2;python_version>='3.4' # BSD
+sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2 # BSD
 openstackdocstheme>=1.20.0 # Apache-2.0
 stestr>=2.0.0
 testtools>=0.9.34

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-minversion = 2.0
-envlist = docs,py27
+minversion = 3.1
+envlist = docs,py37
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
 install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
@@ -11,9 +13,7 @@ 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