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: I56890d9f5d258bc27f94c51c369a774b2978c0fc

tox.ini

@@ -19,6 +19,7 @@ whitelist_externals =
     find
 
 [testenv:tips]
+basepython = python3
 # tests what happens with unreleased version of dependencies, like jenkins-job-builder
 install_command = pip install -U {opts} {packages}
 commands =
@@ -28,6 +29,7 @@ commands =
     stestr run --slowest {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source jenkins --parallel-mode
@@ -38,12 +40,15 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [flake8]
@@ -56,6 +61,7 @@ show-source = True
 exclude = .venv,.tox,dist,doc,build,*.egg
 
 [testenv:bindep]
+basepython = python3
 # Do not install any requirements. We want this to be fast and work even if
 # system dependencies are missing, since it's used to tell you what system
 # dependencies are missing! This also means that bindep must be installed