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: I2db130183c0a657b281e0990593fb86be6bef017
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -17,6 +17,7 @@ whitelist_externals =
   bash
 
 [testenv:pep8]
+basepython = python3
 commands =
   flake8
   # Run security linter
@@ -25,14 +26,17 @@ commands =
   bandit -r keystoneauth1 -x tests -s B110,B410
 
 [testenv:bandit]
+basepython = python3
 # NOTE(browne): This is required for the integration test job of the bandit
 # project. Please do not remove.
 commands = bandit -r keystoneauth1 -x tests -s B110,B410
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source keystoneauth1 --parallel-mode
@@ -43,6 +47,7 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper -t keystoneauth1/tests {posargs}
 
 [flake8]
@@ -62,6 +67,7 @@ import-order-style = pep8
 application-import-names = keystoneauth1
 
 [testenv:docs]
+basepython = python3
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
     -r{toxinidir}/doc/requirements.txt
@@ -71,6 +77,7 @@ commands =
     python setup.py build_sphinx
 
 [testenv:releasenotes]
+basepython = python3
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
     -r{toxinidir}/doc/requirements.txt