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

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

tempest-stress 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: Iea3eb7606ed89ef4ec61ca5d392d44bdfc927c3b

.zuul.yaml

@@ -1,4 +1,3 @@
 - project:
     templates:
-      - openstack-python-jobs
-      - openstack-python3-train-jobs
+      - openstack-python3-ussuri-jobs

releasenotes/notes/drop-py-2-7-6296cbf589c0881c.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Python 2.7 support has been dropped. The minimum version of Python now
+    supported by tempest-stress is Python 3.6.

setup.cfg

@@ -6,6 +6,7 @@ description-file =
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://opendev.org/openstack/tempest-stress/src/branch/master/README.rst
+python-requires = >=3.6
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -13,8 +14,6 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     Programming Language :: Python
-    Programming Language :: Python :: 2
-    Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-minversion = 2.0
-envlist = py36,py37,py27,pypy,pep8
+minversion = 3.1.1
+envlist = py36,py37,pypy,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 setenv =
    VIRTUAL_ENV={envdir}
@@ -15,23 +17,19 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = stestr --test-path ./tempest_stress/tests run {posargs}
 
 [testenv:stress]
-basepython = python3
 envdir = .tox/tempest_stress
 sitepackages = False
 commands =
     run-tempest-stress {posargs}
 
 [testenv:pep8]
-basepython = python3
 sitepackages = False
 commands = flake8 {posargs}
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 setenv =
     VIRTUAL_ENV={envdir}
     PYTHON=coverage run --source tempest_stress --parallel-mode
@@ -42,11 +40,9 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:debug]
-basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [flake8]