Update tox to for py36 only for linting

Change-Id: Ie4f06bc1f801648ab904117c662585af3686be3e

tox.ini

@@ -12,13 +12,9 @@ envlist =
 
 minversion = 3.1.0
 
-# ignore basepython for envs that already specify a version
-ignore_basepython_conflict = true
-
 [testenv]
 setenv =
     PIP_INDEX_URL = https://bbpteam.epfl.ch/repository/devpi/simple
-basepython=python3.6
 extras = test
 commands = pytest -n 2 --basetemp={envtmpdir} --cov={envsitepackagesdir}/{[base]name} --cov={envsitepackagesdir}/{[base]morphval} --cov-branch --no-cov-on-fail --html reports/pytest-{envname}.html --self-contained-html tests/ {posargs}
 
@@ -37,6 +33,7 @@ commands =
     twine check {envtmpdir}/dist/*
 
 [testenv:lint]
+basepython=python3.6
 deps =
     pycodestyle
     pydocstyle