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: Ie53c073d62d0adf3627b165f1ad11c02b1927904

tox.ini

@@ -1,9 +1,10 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 envlist = py27,py37,pep8,docs
+ignore_basepython_conflict = True
 
 [testenv]
-install_command = pip install {opts} {packages}
+basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
@@ -11,11 +12,7 @@ deps =
   -r{toxinidir}/doc/requirements.txt
 commands = stestr run --slowest {posargs}
 
-[testenv:py27]
-basepython = python2.7
-
 [testenv:pep8]
-basepython = python3
 deps =
   -r{toxinidir}/test-requirements.txt
 commands =
@@ -24,11 +21,9 @@ commands =
   bandit -r oslo_policy tests -n5
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 whitelist_externals =
   rm
 deps =
@@ -39,7 +34,6 @@ commands =
   sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:cover]
-basepython = python3
 commands = python setup.py test --coverage --coverage-package-name=oslo_policy --testr-args='{posargs}'
 
 [flake8]
@@ -52,7 +46,6 @@ exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 import_exceptions = oslo_policy._i18n
 
 [testenv:releasenotes]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 whitelist_externals =
   rm
@@ -61,7 +54,6 @@ 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