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: I66d77a977b76118d3a7b74f1f5b238370b43a38d
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -15,14 +15,17 @@ commands =
     stestr run {posargs}
 
 [testenv:pep8]
+basepython = python3
 sitepackages = False
 commands =
     flake8 {posargs}
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source coverage2sql --parallel-mode
@@ -37,12 +40,14 @@ commands =
 
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:debug]
 commands = oslo_debug_helper {posargs}
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [flake8]