fix tox python3 overrides

We want to default to running all tox environments under python 3, so
set the basepython value in each environment.

We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.

We do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.

Change-Id: I363f6753b6319ee480cc02765483fa1a7035739f

tox.ini

@@ -12,6 +12,7 @@ setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/test-requirements.txt
 
 [testenv:linters]
+basepython = python3
 # ignore below cases
 # E006 Line too long
 # E010: Do not on same line as it commands
@@ -31,6 +32,7 @@ commands =
             -print0 | xargs -0 yamllint"
 
 [testenv:pep8]
+basepython = python3
 usedevelop = False
 skip_install = True
 deps =
@@ -39,4 +41,5 @@ commands =
     pep8
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}