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: If6edee698b2a4f0ba5afc13e89462d3b38d753e1

tox.ini

@@ -4,6 +4,7 @@ envlist = pypy,py35,py27,pep8,docs
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv =
@@ -18,6 +19,7 @@ commands =
     coverage erase
 
 [testenv:docs]
+basepython = python3
 deps =
     doc8
     sphinx
@@ -26,21 +28,25 @@ commands =
     doc8 --allow-long-titles doc/source
 
 [testenv:pep8]
+basepython = python3
 commands =
     flake8 {posargs} anchor
     flake8 {posargs} tests
     bandit -r anchor
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands =
     python setup.py testr --coverage --testr-args='{posargs}'
     coverage combine
     coverage xml
 
 [testenv:bandit]
+basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands = bandit -r anchor