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: I36ef5b39b48e259b51f2bb03caf3443d8969c1e9

tox.ini

@@ -1,20 +1,17 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 envlist = py27,py37,pep8
+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
  -r{toxinidir}/requirements.txt
 commands = stestr run --slowest {posargs}
 
-[testenv:py27]
-basepython = python2.7
-
 [testenv:pep8]
-basepython = python3
 deps =
   -r{toxinidir}/test-requirements.txt
 commands =
@@ -23,11 +20,9 @@ commands =
   bandit -r oslo_i18n -x tests -n5
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 whitelist_externals =
   rm
 deps =
@@ -38,7 +33,6 @@ commands =
   sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:cover]
-basepython = python3
 setenv =
     PYTHON=coverage run --source oslo_i18n --parallel-mode
 commands =
@@ -65,7 +59,6 @@ import_exceptions =
     oslo_i18n._i18n._
 
 [testenv:releasenotes]
-basepython = python3
 whitelist_externals =
   rm
 commands =
@@ -73,7 +66,6 @@ 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