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: I5314a982b415866c5fddae8bdcf5f57add08d555
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -36,9 +36,11 @@ commands =
   bandit -r oslo_cache -x tests -n5
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 whitelist_externals = rm
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
@@ -46,6 +48,7 @@ commands =
     sphinx-build -b html doc/source doc/build/html
 
 [testenv:cover]
+basepython = python3
 commands = python setup.py testr --coverage --testr-args='{posargs}'
 
 [flake8]
@@ -58,6 +61,7 @@ exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 import_exceptions =
 
 [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