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: Ie4fd0a69ea0c2ffeff59a2ead1012a730e47084c
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -10,6 +10,7 @@ setenv =
 deps = -r{toxinidir}/test-requirements.txt
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:doc8]
@@ -17,12 +18,12 @@ commands =
   doc8 -e .rst doc/source
 
 [testenv:docs]
+basepython = python3
 commands=
     sphinx-build doc/source doc/build/html
 
 [testenv:pep8]
-# This runs all lint like jobs
-basepython = python2.7
+basepython = python3
 deps =
   flake8
   {[testenv]deps}