Enables unit tests in tox

Unit tests should be also ran in Continous Integration for both Python2
and Python3.
Also fixes `invalid_combinations` list as `python3` recipe was removed
in a19e884.

Author: AndreMiras

tests/test_graph.py

@@ -24,8 +24,7 @@
 def test_valid_recipe_order_and_bootstrap(names, bootstrap):
     get_recipe_order_and_bootstrap(ctx, names, bootstrap)
 
-invalid_combinations = [[['python2', 'python3crystax'], None],
-                        [['python3'], Bootstrap.get_bootstrap('pygame', ctx)]]
+invalid_combinations = [[['python2', 'python3crystax'], None],]
 
 
 @pytest.mark.parametrize('names,bootstrap', invalid_combinations)

tox.ini

@@ -1,7 +1,9 @@
 [tox]
-envlist = pep8
-# no setup.py to be ran
-skipsdist = True
+envlist = pep8,py27,py3
+
+[testenv]
+deps = pytest
+commands = pytest tests/
 
 [testenv:pep8]
 deps = flake8