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: I3868de365369664b3689582cc9d25f3228e5dbb7
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -25,6 +25,7 @@ whitelist_externals = find
                       rm
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper -t troveclient/tests {posargs}
 
 [testenv:debug-py27]
@@ -36,12 +37,15 @@ basepython = python3.5
 commands = oslo_debug_helper -t troveclient/tests {posargs}
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands =
     coverage erase
     python setup.py testr --coverage --testr-args='{posargs}'
@@ -49,6 +53,7 @@ commands =
     coverage report
 
 [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
@@ -58,6 +63,7 @@ commands =
   sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:releasenotes]
+basepython = python3
 deps =
        -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
        -r{toxinidir}/requirements.txt