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: I051f1c18b719a27372b626d483e47327085dd3b7
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -24,6 +24,7 @@ commands =
   {toxinidir}/tools/fast8.sh
 
 [testenv:pep8]
+basepython = python3
 commands =
     flake8
     bandit -r openstackclient -x tests -s B105,B106,B107,B401,B404,B603,B606,B607,B110,B605,B101
@@ -87,6 +88,7 @@ commands =
     {toxinidir}/openstackclient/tests/functional/run_stestr.sh {posargs}
 
 [testenv:venv]
+basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
   -r{toxinidir}/requirements.txt
@@ -94,6 +96,7 @@ deps =
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv =
     VIRTUAL_ENV={envdir}
     PYTHON=coverage run --source openstackclient --parallel-mode
@@ -109,6 +112,7 @@ commands =
     oslo_debug_helper -t openstackclient/tests {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
@@ -118,6 +122,7 @@ commands =
   sphinx-build -a -E -W -d doc/build/doctrees -b man doc/source doc/build/man
 
 [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