cast the value to a bool so pytest doesn't try to eval it

setuptools/tests/test_namespaces.py

@@ -13,7 +13,7 @@ class TestNamespaces:
 
     @pytest.mark.xfail(sys.version_info < (3, 3),
         reason="Requires PEP 420")
-    @pytest.mark.skipif(os.environ.get("APPVEYOR"),
+    @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):
         """