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: I3edccb24a3f714be8fcc32851342363e44f7ea2f
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -23,6 +23,7 @@ commands = stestr run {posargs}
            stestr slowest
 
 [testenv:examples]
+basepython = python3
 commands = stestr --test-path ./openstack/tests/examples run {posargs}
            stestr slowest
 
@@ -35,6 +36,7 @@ commands = stestr --test-path ./openstack/tests/functional run --serial {posargs
            stestr slowest
 
 [testenv:pep8]
+basepython = python3
 usedevelop = False
 skip_install = True
 deps =
@@ -49,15 +51,18 @@ commands =
     flake8
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:debug]
+basepython = python3
 whitelist_externals = find
 commands =
     find . -type f -name "*.pyc" -delete
     oslo_debug_helper {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source openstack --parallel-mode
@@ -77,6 +82,7 @@ deps =
 commands = {toxinidir}/extras/run-ansible-tests.sh -e {envdir} {posargs}
 
 [testenv:docs]
+basepython = python3
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
     -r{toxinidir}/requirements.txt
@@ -84,6 +90,7 @@ deps =
 commands = sphinx-build -W -d doc/build/doctrees -b html doc/source/ doc/build/html
 
 [testenv:releasenotes]
+basepython = python3
 usedevelop = False
 skip_install = True
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html