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: Ieea99ee85a01061eed68d45fc62f24dbffb98076

tox.ini

@@ -19,9 +19,11 @@ commands =
     py.test -vv {posargs:devops/tests}
 
 [testenv:venv]
+basepython = python3
 commands = {posargs:}
 
 [testenv:cover]
+basepython = python3
 deps =
     pytest-cov
     -r{toxinidir}/test-requirements.txt
@@ -33,12 +35,14 @@ commands =
 
 
 [testenv:pep8]
+basepython = python3
 deps = hacking==0.10.1
 usedevelop = False
 commands =
     flake8
 
 [testenv:pylint]
+basepython = python3
 deps =
     -r{toxinidir}/test-requirements.txt
     pylint
@@ -46,12 +50,14 @@ commands =
     pylint --rcfile=.pylintrc_gerrit devops bin/dos.py
 
 [testenv:pylint-py27]
+basepython = python2.7
 deps=
     -r{toxinidir}/test-requirements.txt
     pylint
 commands=pylint devops bin/dos.py
 
 [testenv:pylint-py35]
+basepython = python3
 deps=
     -r{toxinidir}/test-requirements.txt
     pylint
@@ -67,4 +73,5 @@ count = True
 DJANGO_SETTINGS_MODULE=devops.test_settings
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx