tox: Trivial cleanup

Move 'basepython' to the top-level 'testenv'.

Change-Id: I2ab1eb99a61695bfbc7ecc66f215b1cb991007dd

tox.ini

@@ -1,8 +1,10 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 envlist = py27,py37,pep8
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 install_command = pip install {opts} {packages}
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
@@ -14,11 +16,7 @@ commands =
   lockutils-wrapper stestr run --slowest {posargs}
   env TEST_EVENTLET=1 lockutils-wrapper stestr run --slowest {posargs}
 
-[testenv:py27]
-basepython = python2.7
-
 [testenv:pep8]
-basepython = python3
 deps =
   -r{toxinidir}/test-requirements.txt
 commands =
@@ -27,11 +25,9 @@ commands =
   bandit -r oslo_concurrency -x tests -n5 --skip B311,B404,B603,B606
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 whitelist_externals = rm
 deps =
   {[testenv]deps}
@@ -41,7 +37,6 @@ commands =
   sphinx-build -W --keep-going -b html doc/source doc/build/html {posargs}
 
 [testenv:cover]
-basepython = python3
 setenv =
   PYTHON=coverage run --source oslo_concurrency --parallel-mode
 commands =
@@ -60,13 +55,11 @@ import_exceptions =
   oslo_concurrency._i18n
 
 [testenv:releasenotes]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 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