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: I4b329f3233ce126fb25bff6c947aa8379b991d59
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -4,7 +4,6 @@ distribute = False
 envlist = py35,py27,pep8
 
 [testenv]
-basepython = python3
 install_command = pip install {opts} {packages}
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
@@ -15,6 +14,7 @@ commands =
   coverage report --show-missing
 
 [testenv:lower-constraints]
+basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt
@@ -24,20 +24,24 @@ deps =
 basepython = python2.7
 
 [testenv:pep8]
+basepython = python3
 commands =
   flake8
   # Run security linter
   bandit -r oslo_config -x tests -n5
 
 [testenv:cover]
+basepython = python3
 commands =
   python setup.py test --coverage --coverage-package-name=oslo_config --testr-args='{posargs}'
   coverage report --show-missing
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 whitelist_externals = rm
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
@@ -45,6 +49,7 @@ commands =
     sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:bandit]
+basepython = python3
 commands = bandit -r oslo_config -x tests -n5
 
 [flake8]
@@ -53,6 +58,7 @@ enable-extensions = H203,H106
 exclude = .tox,dist,doc,*.egg,build
 
 [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