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: Ifa267fcd8eb5334672dd0b603d42dedb67bd2c98

tox.ini

@@ -20,11 +20,13 @@ commands =
     stestr slowest
 
 [testenv:pep8]
+basepython = python3
 sitepackages = False
 commands =
   flake8 {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv =
     VIRTUAL_ENV={envdir}
     PYTHON=coverage run --source subunit2sql --parallel-mode
@@ -34,9 +36,11 @@ commands =
   coverage html -d cover
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 
@@ -50,10 +54,12 @@ ignore = E125,E123,E129,H305,E711
 exclude = .venv,.git,.tox,dist,doc,*egg,build,releasenotes
 
 [testenv:generate_samples]
+basepython = python3
 commands =
     oslo-config-generator --config-file config-generator/subunit2sql.conf
     oslo-config-generator --config-file config-generator/sql2subunit.conf
     oslo-config-generator --config-file config-generator/subunit2sql-db-manage.conf
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html