Default testenv to basepython = python3

Change-Id: Ifa6ebc07d4edaa8d92cb5f3a8ae9c4ffead38974

tox.ini

@@ -4,6 +4,7 @@ envlist = py27,py37,pypy,pep8
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 setenv =
    VIRTUAL_ENV={envdir}
@@ -17,27 +18,22 @@ deps =
 commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
-basepython = python3
 commands = flake8
            {toxinidir}/tools/coding-checks.sh --all '{posargs}'
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 commands = python setup.py test --coverage --testr-args='{posargs}'
 
 [testenv:docs]
-basepython = python3
 deps =
   -r{toxinidir}/doc/requirements.txt
 commands =
   sphinx-build -W -b html -d doc/build/doctrees doc/source doc/build/html
 
 [testenv:pdf-docs]
-basepython = python3
 envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 whitelist_externals =
@@ -47,18 +43,15 @@ commands =
   make -C doc/build/pdf
 
 [testenv:releasenotes]
-basepython = python3
 deps =
   -r{toxinidir}/doc/requirements.txt
 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}
 
 [testenv:functional]
-basepython = python3
 setenv = {[testenv]setenv}
          OS_TEST_PATH=./ovsdbapp/tests/functional
          OVS_SRCDIR={envdir}/src/ovs
@@ -76,7 +69,6 @@ ignore = E123,E125
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt