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: I78f552cfb434b6bac30a77c5f2011636308fbbee

tox.ini

@@ -24,6 +24,7 @@ whitelist_externals =
     find
 
 [testenv:tips]
+basepython = python3
 # tests what happens with unreleased version of dependencies, like python-jenkins
 install_command = pip install -U {opts} {packages}
 commands =
@@ -33,6 +34,7 @@ commands =
     stestr run --slowest {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source jenkins_jobs --parallel-mode
@@ -43,31 +45,39 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 
 [testenv:pyflakes]
+basepython = python3
 deps = pyflakes
 commands = pyflakes jenkins_jobs tests setup.py
 
 [testenv:compare-xml-config]
+basepython = python3
 commands = jenkins-jobs {posargs:test -o .test/run-conf/default/out/ .test/run-conf/config/}
 
 [testenv:compare-xml-old]
+basepython = python3
 commands = jenkins-jobs test -o .test/old/out/ .test/old/config/
 
 [testenv:compare-xml-new]
+basepython = python3
 commands = jenkins-jobs test -o .test/new/out/ .test/new/config/
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx {posargs}
 
 [testenv:docs-linkcheck]
+basepython = python3
 # If you are behind a proxy, for this test to work you will need to set
 # TOX_TESTENV_PASSENV="http_proxy https_proxy no_proxy ..." to pass
 # through the proxy environment settings to be able to validate any urls.
 commands = python setup.py build_sphinx -b linkcheck
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [flake8]