Expect failure on Python 3.4 and earlier as module_from_spec isn't available. Ref #250.

setuptools/tests/test_namespaces.py

@@ -11,8 +11,8 @@ from . import namespaces
 
 class TestNamespaces:
 
-    @pytest.mark.xfail(sys.version_info < (3, 3),
-        reason="Requires PEP 420")
+    @pytest.mark.xfail(sys.version_info < (3, 5),
+        reason="Requires importlib.util.module_from_spec")
     @pytest.mark.skipif(bool(os.environ.get("APPVEYOR")),
         reason="https://github.com/pypa/setuptools/issues/851")
     def test_mixed_site_and_non_site(self, tmpdir):