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: Ic86d838e3e0093d41d1230f96c5bd85fcc0e8016
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -31,11 +31,13 @@ application-import-names = ironic_lib
 enable-extensions=H106,H203,H204,H205,H210,H904
 
 [testenv:pep8]
+basepython = python3
 commands =
     flake8 {posargs}
     doc8 README.rst doc/source --ignore D001
 
 [testenv:cover]
+basepython = python3
 setenv = VIRTUALENV={envdir}
          LANGUAGE=en_US
          PYTHON=coverage run --source ironic_lib --omit='*tests*' --parallel-mode
@@ -47,9 +49,11 @@ commands =
   coverage html -d ./cover --omit='*tests*'
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 setenv = PYTHONHASHSEED=0
 sitepackages = False
 envdir = {toxworkdir}/venv