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: I01e8cf1b70d7a1501dccd26bf30db52e36dedb38
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -18,6 +18,7 @@ commands = ostestr {posargs}
 whitelist_externals = ostestr
 
 [testenv:unit-tips]
+basepython = python3
 commands =
     pip install -q -e "git+file://{toxinidir}/../cliff#egg=cliff"
     pip install -q -e "git+file://{toxinidir}/../keystoneauth#egg=keystoneauth"
@@ -28,23 +29,29 @@ commands =
 whitelist_externals = ostestr
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands =
     python setup.py test --coverage --coverage-package-name=osc_lib --testr-args='{posargs}'
     coverage report
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper -t osc_lib/tests {posargs}
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [flake8]