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

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

Watcher 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

Depends-On: https://review.opendev.org/#/c/693631/

Change-Id: I603c6d2c22779e8ef2e70eb6369fc521a77c9c3a

tox.ini

@@ -1,9 +1,10 @@
 [tox]
 minversion = 2.0
-envlist = py36,py37,py27,pep8
+envlist = py36,py37,pep8
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 whitelist_externals = find
                       rm
@@ -21,14 +22,12 @@ commands =
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
 [testenv:pep8]
-basepython = python3
 commands =
     doc8 doc/source/ CONTRIBUTING.rst HACKING.rst README.rst
     flake8
     bandit -r watcher -x watcher/tests/* -n5 -ll -s B320
 
 [testenv:venv]
-basepython = python3
 setenv = PYTHONHASHSEED=0
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
@@ -38,7 +37,6 @@ deps =
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 setenv =
     PYTHON=coverage run --source watcher --parallel-mode
 commands =
@@ -49,7 +47,6 @@ commands =
     coverage report
 
 [testenv:docs]
-basepython = python3
 setenv = PYTHONHASHSEED=0
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
@@ -57,7 +54,6 @@ commands =
   sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:api-ref]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 whitelist_externals = bash
 commands =
@@ -65,17 +61,14 @@ commands =
   sphinx-build -W --keep-going -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
 
 [testenv:debug]
-basepython = python3
 commands = oslo_debug_helper -t watcher/tests {posargs}
 
 [testenv:genconfig]
-basepython = python3
 sitepackages = False
 commands =
     oslo-config-generator --config-file etc/watcher/oslo-config-generator/watcher.conf
 
 [testenv:genpolicy]
-basepython = python3
 commands =
     oslopolicy-sample-generator --config-file etc/watcher/oslo-policy-generator/watcher-policy-generator.conf
 
@@ -88,7 +81,6 @@ enable-extensions = H106,H203,H904
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build,*sqlalchemy/alembic/versions/*,demo/,releasenotes
 
 [testenv:wheel]
-basepython = python3
 commands = python setup.py bdist_wheel
 
 [hacking]
@@ -101,7 +93,6 @@ extension=.rst
 ignore-path=doc/source/image_src,doc/source/man,doc/source/api
 
 [testenv:pdf-docs]
-basepython = python3
 envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 whitelist_externals =
@@ -113,17 +104,14 @@ commands =
   make -C doc/build/pdf
 
 [testenv:releasenotes]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -W -E -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
 
 [testenv:bandit]
-basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands = bandit -r watcher -x watcher/tests/* -n5 -ll -s B320
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt