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: I29139cd2cb8e6296cec362cac1c1f23c7561115f
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -33,6 +33,7 @@ setenv =
 
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands=
     bash -c "rm -rf doc/build"
@@ -46,6 +47,7 @@ extensions = .rst
 
 
 [testenv:releasenotes]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
     sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
@@ -53,11 +55,13 @@ commands =
 
 # environment used by the -infra templated docs job
 [testenv:venv]
+basepython = python3
 commands =
     {posargs}
 
 
 [testenv:pep8]
+basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-pep8.sh"
 
@@ -90,6 +94,7 @@ commands =
 
 
 [testenv:linters]
+basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-ansible-env-prep.sh"
     {[testenv:pep8]commands}