Drop python 2.7 support and testing

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

Change-Id: Iff873ae15f865aa8740760a3e904a96d22e553e8

.zuul.yaml

@@ -4,7 +4,6 @@
       - lib-forward-testing
       - lib-forward-testing-python3
       - openstack-lower-constraints-jobs
-      - openstack-python-jobs
       - openstack-python3-ussuri-jobs
       - osc-tox-unit-tips
       - publish-openstack-docs-pti

setup.cfg

@@ -13,8 +13,6 @@ 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 +1,10 @@
 [tox]
 minversion = 2.0
-envlist = py27,py37,pep8
+envlist = py37,pep8
 skipdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
@@ -18,7 +19,6 @@ commands = stestr run {posargs}
 whitelist_externals = stestr
 
 [testenv:unit-tips]
-basepython = python3
 commands =
     pip install -q -e "git+file://{toxinidir}/../cliff#egg=cliff"
     pip install -q -e "git+file://{toxinidir}/../keystoneauth#egg=keystoneauth"
@@ -28,25 +28,20 @@ commands =
 whitelist_externals = stestr
 
 [testenv:pep8]
-basepython = python3
 commands = flake8
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 commands =
     python setup.py test --coverage --coverage-package-name=osc_lib --testr-args='{posargs}'
     coverage report
 
 [testenv:debug]
-basepython = python3
 commands = oslo_debug_helper -t osc_lib/tests {posargs}
 
 [testenv:docs]
-basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/upper-constraints.txt}
   -r{toxinidir}/requirements.txt
@@ -58,7 +53,6 @@ commands =
     sphinx-build -a -E -W -d doc/build/doctrees -b html doc/source doc/build/html
 
 [testenv:releasenotes]
-basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [flake8]
@@ -69,7 +63,6 @@ exclude = .git,.tox,dist,doc,*lib/python*,*egg,build,tools
 ignore = __
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt