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: I24b87db5bd80876ff631698350606aa4e23919ee
Closes-Bug:  #1801657

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?h=stable/pike} {opts} {packages}
 setenv =
     VIRTUAL_ENV={envdir}
@@ -50,10 +49,12 @@ basepython = python3.4
 commands = oslo_debug_helper {posargs}
 
 [testenv:pep8]
+basepython = python3
 deps = hacking<0.11,>=0.10.2
 commands = flake8
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
@@ -84,6 +85,7 @@ commands =
 whitelist_externals = rm
 
 [testenv:newnote]
+basepython = python3
 # Re-use the releasenotes venv
 envdir = {toxworkdir}/releasenotes
 deps = -r{toxinidir}/doc/requirements.txt
@@ -99,6 +101,7 @@ commands =
 whitelist_externals = rm
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [flake8]
@@ -109,6 +112,7 @@ builtins = _
 exclude=.venv,.git,.tox,dist,doc,*openstack/common*,*lib/python*,*egg,build
 
 [testenv:genconfig]
+basepython = python3
 commands =
     oslo-config-generator --config-file=tools/config-generator.conf
 
@@ -125,6 +129,7 @@ commands =
      pylint {posargs} dcmanager dcorch --rcfile=./pylint.rc 
 
 [testenv:linters]
+basepython = python3
 # bashate ignore:
 #  E006 - accept long lines
 #  E040 - false positive on |& syntax (new in bash 4)