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: If352b0ca1de2cda56089ebe1c7db04feb5ba983b
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -25,6 +25,7 @@ commands =
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
 [testenv:docs]
+basepython = python3
 deps =
        -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
        -r{toxinidir}/requirements.txt
@@ -32,18 +33,23 @@ deps =
 commands = sphinx-build -b html doc/source doc/build/html
 
 [testenv:flake8]
+basepython = python3
 commands = flake8
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 
 [testenv:pyflakes]
+basepython = python3
 commands = flake8
 
 [testenv:cover]
+basepython = python3
 commands = python setup.py testr --coverage --testr-args='{posargs}'
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:functional]
@@ -57,6 +63,7 @@ passenv = OS_STDOUT_CAPTURE
           TEMPEST_CONFIG
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [flake8]