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: I383a87c917dcdf9469d57b55ab2630420e3ac87c
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -55,15 +55,18 @@ commands =
   stestr run {posargs}
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:pep8]
+basepython = python3
 commands =
   flake8
   neutron-db-manage --subproject neutron-dynamic-routing --database-connection sqlite:// check_migration
   {[testenv:genconfig]commands}
 
 [testenv:cover]
+basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source neutron_dynamic_routing --parallel-mode
@@ -74,9 +77,11 @@ commands =
   coverage xml -o cover/coverage.xml
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [flake8]