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: I1bd6a3aebbbe539d4f21ca71c76d92e3c325c1e8
Closes-Bug:  #1802032

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
 whitelist_externals = bash
 #bashate ignore errors
 #E010: do not on the same line as for
@@ -33,6 +33,7 @@ commands =
             -print0 | xargs -0 yamllint"
 
 [testenv:pep8]
+basepython = python3
 usedevelop = False
 deps =
     flake8<3.6.0
@@ -71,6 +72,7 @@ commands =
 whitelist_externals = rm
 
 [testenv:newnote]
+basepython = python3
 # Re-use the releasenotes venv
 envdir = {toxworkdir}/releasenotes
 deps = -r{toxinidir}/doc/requirements.txt