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: I7a3be998629ea8ea56ede75bdd05380019cc9372

tox.ini

@@ -21,10 +21,12 @@ deps =
 basepython = python2.7
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 skip_install = True
 
 [testenv:cover]
+basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source shade --parallel-mode
@@ -35,9 +37,11 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:venv]
+basepython = python3
 usedevelop = true
 commands = {posargs}