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: I98b578898a34f94bbe8de7bb02cd2c94626eaac7
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -3,7 +3,6 @@ minversion = 2.0
 envlist = py35,py27,pep8
 
 [testenv]
-basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
   -r{toxinidir}/test-requirements.txt
@@ -15,6 +14,7 @@ commands = python setup.py testr --slowest --testr-args='{posargs}'
 basepython = python2.7
 
 [testenv:pep8]
+basepython = python3
 deps =
   -r{toxinidir}/test-requirements.txt
 commands =
@@ -23,12 +23,15 @@ commands =
   bandit -r oslo_reports tests -n5 --skip B314,B405
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:cover]
+basepython = python3
 commands = python setup.py test --coverage --coverage-package-name=oslo_reports --testr-args='{posargs}'
 
 [flake8]
@@ -42,9 +45,11 @@ exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 import_exceptions =
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:lower-constraints]
+basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt