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: I450085ff6d9cc352ff38798c361cbe2fbebce263
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -21,15 +21,18 @@ commands =
  python setup.py testr --slowest --testr-args='{posargs} --concurrency=1'
 
 [testenv:pep8]
+basepython = python3
 usedevelop = False
 deps =
 commands = {toxinidir}/tools/pip-install-single-req.sh test-requirements.txt hacking
            flake8
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands = 
   coverage erase
   find . -type f -name "*.pyc" -delete
@@ -37,9 +40,11 @@ commands =
   coverage report
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper -t congress/tests {posargs}
 
 [testenv:bench]
+basepython = python3
 setenv =
    TEST_BENCHMARK=true
    VIRTUAL_ENV={envdir}
@@ -55,15 +60,18 @@ builtins = _
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build,*thirdparty/*,CongressLexer.py,CongressParser.py,contrib/*
 
 [testenv:genconfig]
+basepython = python3
 deps =
 commands = {toxinidir}/tools/pip-install-single-req.sh requirements.txt oslo.config
            oslo-config-generator --config-file=etc/congress-config-generator.conf
            oslo-config-generator --config-file=etc/congress-agent-config-generator.conf
 
 [testenv:genpolicy]
+basepython = python3
 commands = oslopolicy-sample-generator --config-file etc/congress-policy-generator.conf
 
 [testenv:docs]
+basepython = python3
 setenv = PYTHONHASHSEED=0
 deps = -r{toxinidir}/doc/requirements.txt
        -r{toxinidir}/requirements.txt
@@ -71,10 +79,12 @@ commands = rm -rf doc/build doc/source/api
            sphinx-build -W -b html doc/source doc/build/html
 
 [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
 
 [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