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: I64d454d251712281911a1aa6e4ecd6139ad1bd8b

tox.ini

@@ -16,14 +16,17 @@ passenv=
     DUMP_CONFIG_GRAPH
 
 [testenv:pep8]
+basepython = python3
 commands =
   flake8 {posargs}
   dib-lint
 
 [testenv:pylint]
+basepython = python3
 commands = pylint --rcfile pylint.cfg diskimage_builder
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:func]
@@ -34,12 +37,15 @@ commands = {toxinidir}/tests/run_functests.sh {posargs}
 # NOTE: this is "setup.py test" (*not* testr) which is a pbr wrapper
 # around testr.  This understands --coverage-package-name which we
 # need due to underscore issues.
+basepython = python3
 commands = python setup.py test --coverage --coverage-package-name diskimage_builder --testr-args='{posargs}'
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -W -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [flake8]