Removing NO_MORE_PAGES sentinel and just making None mean no more pages.

Also renaming next_page() to update_page() on Iterator and dropping
any return value from that method. Also throwing an AttributeError
if the page is unset on @Property access.

packages/google-cloud-resource-manager/unit_tests/test_client.py

@@ -68,7 +68,7 @@ def dummy_response():
         iterator = self._makeOne(client)
         iterator._get_next_page_response = dummy_response
 
-        iterator.next_page()
+        iterator.update_page()
         page = iterator.page
         self.assertEqual(page.num_items, 1)
         project = iterator.next()