tox: Trivial cleanup

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

Change-Id: I4957925d96c2eea0f9a2d791feb71b6d3f54a224

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 distribute = False
 envlist = py27,py37,pep8
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 whitelist_externals =
   find
 setenv =
@@ -21,25 +23,19 @@ commands =
   stestr slowest
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 
-[testenv:py27]
-basepython = python2.7
-
 [testenv:pep8]
-basepython = python3
 commands =
   flake8
   # Run security linter
   bandit -r oslo_config -x tests -n5
 
 [testenv:cover]
-basepython = python3
 setenv =
     PYTHON=coverage run --source oslo_config --parallel-mode
 commands =
@@ -51,11 +47,9 @@ commands =
   coverage report --show-missing
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 whitelist_externals = rm
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
@@ -63,7 +57,6 @@ commands =
   sphinx-build -W --keep-going -b html doc/source doc/build/html {posargs}
 
 [testenv:bandit]
-basepython = python3
 commands = bandit -r oslo_config -x tests -n5
 
 [flake8]
@@ -72,7 +65,6 @@ enable-extensions = H203,H106
 exclude = .tox,dist,doc,*.egg,build
 
 [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