tox: Remove 'ignore_base_python_conflict'

We don't need this in a world where Python 3 is the default Python
version in virtually all environments.

We also reshuffle the file to make the ordering a little more logical.

Change-Id: I1c207c4310b9617770e6097ce9a583789f7ce85b
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

tox.ini

@@ -1,16 +1,23 @@
 [tox]
 minversion = 3.18.0
 envlist = py3,pep8
-ignore_basepython_conflict = true
 
 [testenv]
-basepython = python3
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
 commands = stestr run --slowest {posargs}
 
+[testenv:cover]
+setenv =
+  PYTHON=coverage run --source oslo_i18n --parallel-mode
+commands =
+  stestr run {posargs}
+  coverage combine
+  coverage html -d cover
+  coverage xml -o cover/coverage.xml
+
 [testenv:pep8]
 deps =
   -r{toxinidir}/test-requirements.txt
@@ -32,32 +39,6 @@ commands =
   rm -rf doc/build
   sphinx-build -W --keep-going -b html doc/source doc/build/html
 
-[testenv:cover]
-setenv =
-  PYTHON=coverage run --source oslo_i18n --parallel-mode
-commands =
-  stestr run {posargs}
-  coverage combine
-  coverage html -d cover
-  coverage xml -o cover/coverage.xml
-
-
-[flake8]
-# E123, E125 skipped as they are invalid PEP-8.
-# E731 skipped as assign a lambda expression
-
-show-source = True
-ignore = E123,E125,E731,W504
-
-# H106: Don’t put vim configuration in source files
-# H203: Use assertIs(Not)None to check for None
-enable-extensions=H106,H203
-exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build,__init__.py
-
-[hacking]
-import_exceptions =
-  oslo_i18n._i18n._
-
 [testenv:releasenotes]
 whitelist_externals =
   rm
@@ -67,3 +48,17 @@ deps =
 commands =
   rm -rf releasenotes/build
   sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
+
+[flake8]
+show-source = True
+# E123, E125 skipped as they are invalid PEP-8.
+# E731 skipped as assign a lambda expression
+ignore = E123,E125,E731,W504
+# H106: Don’t put vim configuration in source files
+# H203: Use assertIs(Not)None to check for None
+enable-extensions=H106,H203
+exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build,__init__.py
+
+[hacking]
+import_exceptions =
+  oslo_i18n._i18n._