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

google/cloud/storage/batch.py

@@ -214,10 +214,10 @@ def _prepare_batch_request(self):
             timeout = _timeout
 
         # The `email` package expects to deal with "native" strings
-        if six.PY3:  # pragma: NO COVER  Python3
-            buf = io.StringIO()
-        else:
+        if six.PY2:  # pragma: NO COVER  Python3
             buf = io.BytesIO()
+        else:
+            buf = io.StringIO()
         generator = Generator(buf, False, 0)
         generator.flatten(multi)
         payload = buf.getvalue()

tests/unit/test__signing.py

@@ -69,8 +69,8 @@ def test_w_expiration_int(self):
         self.assertEqual(self._call_fut(123), 123)
 
     def test_w_expiration_long(self):
-        if six.PY3:
-            raise unittest.SkipTest("No long on Python 3")
+        if not six.PY2:
+            raise unittest.SkipTest("No long on Python 3+")
 
         self.assertEqual(self._call_fut(long(123)), 123)  # noqa: F821