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: Ia9c73f1f653d7a22354c9478672ba6c5e7a7928c
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -19,23 +19,28 @@ deps =
 commands = pifpaf run aodh -- python setup.py test --slowest --testr-args='{posargs}'
 
 [testenv:pep8]
+basepython = python3
 deps = hacking<0.13,>=0.12
 commands = flake8
 
 [testenv:venv]
+basepython = python3
 # Used by openstack-infra to build the doc
 deps = -r{toxinidir}/doc/requirements.txt
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands =
    python setup.py test --coverage --testr-args='{posargs}'
    coverage report
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
    rm -rf doc/build
@@ -43,6 +48,7 @@ commands =
 whitelist_externals = rm
 
 [testenv:debug]
+basepython = python3
 commands = pifpaf --debug run aodh -- oslo_debug_helper {posargs}
 
 [flake8]