Stop testing python 2 in tox.

Remove python 2 from envlist parameter. Check the link:

https://etherpad.openstack.org/p/drop-python2-support
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

The plan is drop the python 2 support from OpenStack in Ussuri release.
Move the basepython parameter.

Change-Id: I52b974f7c701c8a46001a29bdf5f7b077f898cba

setup.cfg

@@ -16,8 +16,6 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: OS Independent
     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 = pep8,py27,py37,py36
+minversion = 3.2.0
+envlist = pep8,py37,py36
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 passenv =
@@ -21,7 +22,6 @@ deps =
 commands = stestr run {posargs}
 
 [testenv:bindep]
-basepython = python3
 # Do not install any requirements. We want this to be fast and work even if
 # system dependencies are missing, since it's used to tell you what system
 # dependencies are missing! This also means that bindep must be installed
@@ -30,16 +30,13 @@ deps = bindep
 commands = bindep test
 
 [testenv:pep8]
-basepython = python3
 commands = flake8
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 passenv = *
 
 [testenv:cover]
-basepython = python3
 setenv =
   PYTHON=coverage run --source tripleoclient --parallel-mode
   HOME={envdir}
@@ -52,14 +49,12 @@ commands =
   coverage report
 
 [testenv:debug]
-basepython = python3
 deps =
     oslotest
     {[testenv]deps}
 commands = oslo_debug_helper -t tripleoclient/tests {posargs}
 
 [testenv:docs]
-basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
@@ -73,7 +68,6 @@ builtins = _
 exclude=.venv,.git,.tox,dist,doc,*openstack/common*,*lib/python*,*egg,build,releasenotes
 
 [testenv:pdf-docs]
-basepython = python3
 whitelist_externals = make
 description =
   Build PDF documentation.
@@ -84,7 +78,6 @@ commands =
   make -C doc/build/pdf
 
 [testenv:genconfig]
-basepython = python3
 setenv =
    HOME={env:HOME:/home/stack}
 commands =
@@ -93,7 +86,6 @@ commands =
    oslo-config-generator --config-file config-generator/minion.conf
 
 [testenv:releasenotes]
-basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
@@ -103,7 +95,6 @@ commands =
 
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt