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: I7c672f05753886e234430c760f1afc1ed01ff2a3

tox.ini

@@ -1,11 +1,12 @@
 [tox]
-minversion = 1.8
+minversion = 3.1
 envlist = py27,py37,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
-install_command = pip install {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    OS_DEBUG=1
@@ -18,15 +19,12 @@ deps =
 commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
-basepython = python3
 commands = flake8 {posargs}
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 deps = {[testenv]deps}
     coverage
 setenv =
@@ -38,7 +36,6 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-basepython = python3
 whitelist_externals =
   rm
 deps =
@@ -49,7 +46,6 @@ commands =
   sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:debug]
-basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [testenv:lower-constraints]