test: tox allow parallel python versions

tox.ini

 [tox]
 isolated_build = true
-envlist = lint,test
+envlist = lint,py3
 
 [testenv:lint]
 deps =
@@ -8,7 +8,7 @@ deps =
 commands =
     flake8 pycgtool
 
-[testenv:test]
+[testenv]
 deps =
     pytest
     pytest-cov