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: I0fec1df37672b9bdadaa8eeaf8aa7d72f8237a5b
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -12,6 +12,7 @@ commands = bash -c 'if [ ! -d ./.testrepository ] ; then testr init ; fi'
            testr run {posargs}
 
 [testenv:pep8]
+basepython = python3
 setenv =
     PYTHONPATH = .
 commands =
@@ -25,12 +26,14 @@ commands =
     check-uuid --fix --package heat_tempest_plugin
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
        sphinxcontrib-httpdomain
 commands = python setup.py build_sphinx
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [flake8]