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: I972721cf714d811a427ee6a860b0c18c9920c8bc
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -26,6 +26,7 @@ commands = stestr --test-path ./shade/tests/functional run --serial {posargs}
            stestr slowest
 
 [testenv:pep8]
+basepython = python3
 usedevelop = False
 skip_install = True
 deps =
@@ -38,15 +39,18 @@ commands =
     flake8 shade
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:debug]
+basepython = python3
 whitelist_externals = find
 commands =
     find . -type f -name "*.pyc" -delete
     oslo_debug_helper {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source shade --parallel-mode
@@ -57,6 +61,7 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:ansible]
+basepython = python3
 # Need to pass some env vars for the Ansible playbooks
 passenv = HOME USER
 deps =
@@ -67,6 +72,7 @@ deps =
 commands = {toxinidir}/extras/run-ansible-tests.sh -e {envdir} {posargs}
 
 [testenv:docs]
+basepython = python3
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
     -r{toxinidir}/requirements.txt
@@ -75,6 +81,7 @@ commands =
   sphinx-build -W -d doc/build/doctrees -b html doc/source/ doc/build/html
 
 [testenv:releasenotes]
+basepython = python3
 skip_install = True
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}