tox: Trivial cleanup

Move 'basepython' to the top-level 'testenv'.
Use the default 'install_command'

https://tox.readthedocs.io/en/latest/config.html#conf-install_command

Change-Id: I255de2a755b7783f20925d8ca4de6e34e5aee36b

tox.ini

@@ -1,13 +1,14 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 envlist = py27,py37,pep8,bandit
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 setenv =
     VIRTUAL_ENV={envdir}
     BRANCH_NAME=master
     CLIENT_NAME=oslo.service
-install_command = pip install {opts} {packages}
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
@@ -19,18 +20,15 @@ commands =
     stestr run --slowest {posargs}
 
 [testenv:pep8]
-basepython = python3
 commands =
     flake8
     bandit -r oslo_service -n5 -x tests
     doc8 --ignore-path "doc/source/history.rst" doc/source
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 whitelist_externals =
   rm
 commands =
@@ -38,7 +36,6 @@ commands =
   sphinx-build -W --keep-going -b html doc/source doc/build/html {posargs}
 
 [testenv:cover]
-basepython = python3
 commands = python setup.py test --coverage --coverage-package-name=oslo_service --testr-args='{posargs}'
 
 [flake8]
@@ -53,11 +50,9 @@ exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 import_exceptions = oslo_service._i18n
 
 [testenv:debug]
-basepython = python3
 commands = oslo_debug_helper -t oslo_service/tests {posargs}
 
 [testenv:bandit]
-basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
@@ -65,7 +60,6 @@ deps =
 commands = bandit -r oslo_service -n5 -x tests {posargs}
 
 [testenv:releasenotes]
-basepython = python3
 whitelist_externals =
   rm
 commands =
@@ -73,7 +67,6 @@ 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