Run on the same python as tox by default.
[tox]
-envlist = py{27,34,35,36,37}-{native,libs,sa10,sa12,sa13,sa}
+envlist = python{,-libs,-sa10,-sa12,-sa13,-sa}
minversion = 3.2
# https://github.com/jaraco/skeleton/issues/6
tox_pip_extensions_ext_venv_update = true
