tox: Trivial cleanup

move 'basepython' to the top-level 'testenv'

Change-Id: Icd5c18a2eb0d75f015e7ea7ee3c6554055a24798

tox.ini

@@ -1,8 +1,10 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 envlist = py27,py37,pep8
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
  -r{toxinidir}/test-requirements.txt
@@ -10,7 +12,6 @@ deps =
 commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
-basepython = python3
 deps =
   -r{toxinidir}/test-requirements.txt
 commands =
@@ -19,11 +20,9 @@ commands =
   bandit -r oslo_context -x tests -n5
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 whitelist_externals = rm
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
@@ -33,7 +32,6 @@ commands =
   sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:cover]
-basepython = python3
 setenv =
   PYTHON=coverage run --source oslo_context --parallel-mode
 commands =
@@ -53,7 +51,6 @@ exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 import_exceptions =
 
 [testenv:releasenotes]
-basepython = python3
 whitelist_externals = rm
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
@@ -63,7 +60,6 @@ commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt