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: I4660a35ff63f3f0212978dda668ba2cb08774e3a
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -18,12 +18,14 @@ whitelist_externals =
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY PBR_VERSION
 
 [testenv:api-ref]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
   bash -c "rm -rf api-ref/build"
   sphinx-build -W -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
 
 [testenv:pep8]
+basepython = python3
 deps =
      .[bandit]
      {[testenv]deps}
@@ -36,12 +38,14 @@ commands =
   bandit -r keystone -x tests
 
 [testenv:bandit]
+basepython = python3
 # NOTE(browne): This is required for the integration test job of the bandit
 # project. Please do not remove.
 deps = .[bandit]
 commands = bandit -r keystone -x tests
 
 [testenv:cover]
+basepython = python3
 # Also do not run test_coverage_ext tests while gathering coverage as those
 # tests conflict with coverage.
 setenv =
@@ -55,13 +59,16 @@ commands =
   coverage xml -o cover/coverage.xml
 
 [testenv:patch_cover]
+basepython = python3
 commands =
   bash tools/cover.sh
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:debug]
+basepython = python3
 commands =
   find keystone -type f -name "*.pyc" -delete
   oslo_debug_helper {posargs}
@@ -119,6 +126,7 @@ exclude=.venv,.git,.tox,build,dist,*lib/python*,*egg,tools,vendor,.update-venv,*
 max-complexity=24
 
 [testenv:docs]
+basepython = python3
 deps =
   -r{toxinidir}/doc/requirements.txt
   .[ldap,memcache,mongodb]
@@ -128,13 +136,16 @@ commands=
     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:genconfig]
+basepython = python3
 commands = oslo-config-generator --config-file=config-generator/keystone.conf
 
 [testenv:genpolicy]
+basepython = python3
 commands = oslopolicy-sample-generator --config-file config-generator/keystone-policy-generator.conf
 
 [hacking]
@@ -144,6 +155,7 @@ import_exceptions =
 local-check-factory = keystone.tests.hacking.checks.factory
 
 [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