[ussuri][goal] Drop python 2.7 support and testing

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

pymod2pkg is ready with python 3 and ok to drop the
python 2.7 support.

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: I7cf441a76a209287eb935e2d360022d957b56954

.zuul.yaml

@@ -1,6 +1,5 @@
 - project:
     templates:
       - openstack-lower-constraints-jobs
-      - openstack-python-jobs
-      - openstack-python3-train-jobs
+      - openstack-python3-ussuri-jobs
       - publish-openstack-docs-pti

test-requirements.txt

@@ -5,6 +5,5 @@ stestr>=2.0.0 # Apache-2.0
 testresources>=2.0.0 # Apache-2.0/BSD
 testtools>=2.2.0 # MIT
 
-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
+sphinx!=1.6.6,!=1.6.7,>=1.6.2  # BSD
 openstackdocstheme>=1.18.1 # Apache-2.0

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-envlist = py27,py37,pypy,pep8
-minversion = 2.0
+envlist = py37,pypy,pep8
+minversion = 3.1
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
@@ -17,23 +19,19 @@ deps =
 commands = stestr run {posargs}
 
 [testenv:pep8]
-basepython = python3
 deps = {[testenv]deps}
     flake8<3.7.0,>=3.6.0
 commands = flake8
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 commands =
   sphinx-build -W -b html doc/source doc/build/html
 
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt