fix: replace unsafe six.PY3 with PY2 for better future compatibility with Python 4 (#10081)

* fix: fix for Python 4: replace unsafe six.PY3 with PY2

* Fix wording

Author: hugovk

tests/unit/handlers/test__helpers.py

@@ -77,7 +77,7 @@ def get(self):
         self.response.out.write(json.dumps(trace_id))
 
 
-@unittest.skipIf(six.PY3, "webapp2 is Python 2 only")
+@unittest.skipIf(not six.PY2, "webapp2 is Python 2 only")
 class Test_get_trace_id_from_webapp2(unittest.TestCase):
     @staticmethod
     def create_app():