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: If3608222dd55f4f20eed83557d754d867b4e52f0

tox.ini

@@ -4,7 +4,6 @@ minversion = 2.3
 skipsdist = True
 
 [testenv]
-basepython = python3
 install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          OS_STDOUT_CAPTURE=1
@@ -14,6 +13,7 @@ deps = -r{toxinidir}/test-requirements.txt
 whitelist_externals = reno
 
 [testenv:linters]
+basepython = python3
 # bashate ignore:
 #  E006 - accept long lines
 #  E040 - false positive on |& syntax (new in bash 4)
@@ -33,6 +33,7 @@ commands =
          -print0 | xargs -0 bashate -v -iE006,E040"
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
@@ -52,6 +53,7 @@ commands =
 whitelist_externals = rm
 
 [testenv:newnote]
+basepython = python3
 # Re-use the releasenotes venv
 envdir = {toxworkdir}/releasenotes
 deps = -r{toxinidir}/doc/requirements.txt