fix tox python3 overrides

We want to default to running all tox environments under python 3, so
set the basepython value in each environment.

We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.

We do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.

Change-Id: I5078d8e6ab1cadd0d3780a3cd4ebc14c5f05dc69
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -3,7 +3,6 @@ minversion = 2.0
 envlist = py35,py27,pypy,pep8
 
 [testenv]
-basepython = python3
 install_command = pip install {opts} {packages}
 whitelist_externals = find
                       rm
@@ -16,14 +15,17 @@ commands =
     sphinx-build -E -W -b doctest doc/source doc/build/doctest
 
 [testenv:pep8]
+basepython = python3
 commands =
     flake8
     doc8 doc/source
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands =
   coverage erase
   find . -type f -name "*.pyc" -delete
@@ -34,11 +36,13 @@ commands =
 basepython = python2.7
 
 [testenv:docs]
+basepython = python3
 commands =
     rm -rf doc/build
     sphinx-build -E -W -b html doc/source doc/build/html
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [flake8]
@@ -49,9 +53,11 @@ ignore = E123,E125
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:lower-constraints]
+basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt