Move basepython variable to the [testenv]

In tox.ini, while we add a new test class, we need default
depends on python3, this patch move 'basepython = python3'
to the [testenv] as the default configuration.

The py35,py27 in envlist are unused in this project, so I
was also removed that in this patch.

Change-Id: Ib226cbf9fc1fb7d3146083d65c33d9e0b90e4d5b

tox.ini

@@ -1,9 +1,10 @@
 [tox]
 minversion = 2.0
-envlist = py35,py27,pep8
+envlist = docs,pep8
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 setenv =
@@ -17,17 +18,14 @@ deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/
 commands = stestr run {posargs}
 
 [testenv:pep8]
-basepython = python3
 commands =
     flake8 {posargs}
     doc8 specs doc/source
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 setenv =
     VIRTUAL_ENV={envdir}
     PYTHON=coverage run --source specs --parallel-mode
@@ -38,16 +36,13 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:releasenotes]
-basepython = python3
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:debug]
-basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [flake8]
@@ -59,7 +54,6 @@ builtins = _
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 
 [testenv:doc8]
-basepython = python3
 deps =
     -r{toxinidir}/requirements.txt
     doc8