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.

Co-Authored-By: Nguyen Hai <nguyentrihai93@gmail.com>
Change-Id: I5279f9f798c1a9d198e951c8948db78821c7ed7b
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -17,21 +17,26 @@ deps =
 commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 distribute = false
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:releasenotes]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:cover]
+basepython = python3
 setenv =
     PYTHON=coverage run --source tackerclient --parallel-mode
 commands =