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: I21491f302cbc6ca7a9b5ac12c487cf214bf4866e
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -20,19 +20,23 @@ deps = -r{toxinidir}/requirements.txt
 commands = stestr run {posargs}
 
 [testenv:pep8]
+basepython = python3
 commands =
   flake8
   bandit -r keystonemiddleware -x tests -n5
 
 [testenv:bandit]
+basepython = python3
 # NOTE(browne): This is required for the integration test job of the bandit
 # project. Please do not remove.
 commands = bandit -r keystonemiddleware -x tests -n5
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source keystonemiddleware --parallel-mode
@@ -43,6 +47,7 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:debug]
+basepython = python3
 
 commands = oslo_debug_helper -t keystonemiddleware/tests {posargs}
 
@@ -58,12 +63,14 @@ show-source = True
 exclude = .venv,.tox,dist,doc,*egg,build
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands=
     doc8 doc/source
     python setup.py build_sphinx
 
 [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