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: I4f96e60a45c9ca93e9e3cd1951900371f20bb06d

tox.ini

@@ -2,6 +2,7 @@
 envlist = py27,pep8
 
 [testenv]
+basepython = python3
 distribute = False
 setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/test-requirements.txt
@@ -11,27 +12,33 @@ sitepackages = False
 [tox:jenkins]
 
 [testenv:style]
+basepython = python3
 deps = flake8
        setuptools_git>=0.4
 commands = flake8 astara setup.py
 
 [testenv:pep8]
+basepython = python3
 deps = {[testenv:style]deps}
 commands = {[testenv:style]commands}
 
 [testenv:doc]
+basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands =
     sphinx-build doc/source doc/build
     sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:cover]
+basepython = python3
 setenv = NOSE_WITH_COVERAGE=1
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:functional]
+basepython = python3
 commands = nosetests -v ./astara/test/functional/
 
 [flake8]
@@ -40,7 +47,9 @@ exclude=.venv,.git,.tox,dist,doc,.idea
 show-source = True
 
 [testenv:genconfig]
+basepython = python3
 commands = {toxinidir}/tools/generate_config_file_samples.sh
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html