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: Ie7da78c0be12708510b9e12a4844b58c7e1ddc2a
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -11,6 +11,7 @@ deps = -r{toxinidir}/test-requirements.txt
 whitelist_externals = bash
 
 [testenv:bindep]
+basepython = python3
 # Do not install any requirements. We want this to be fast and work even if
 # system dependencies are missing, since it's used to tell you what system
 # dependencies are missing! This also means that bindep must be installed
@@ -19,6 +20,7 @@ deps = bindep
 commands = bindep test
 
 [testenv:pep8]
+basepython = python3
 commands =
     # Run hacking/flake8 check for all python files
     bash -c "git ls-files | grep -v releasenotes |  xargs grep --binary-files=without-match \
@@ -38,6 +40,7 @@ commands =
   bash ci-scripts/ansible-lint.sh
 
 [testenv:linters]
+basepython = python3
 deps =
     -r{toxinidir}/test-requirements.txt
     -r{toxinidir}/ansible-requirements.txt
@@ -46,10 +49,12 @@ commands =
     {[testenv:ansible-lint]commands}
 
 [testenv:releasenotes]
+basepython = python3
 whitelist_externals = bash
 commands = bash -c ci-scripts/releasenotes_tox.sh
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [flake8]