Fixed descriptor_pool_test.py for Python 2.6

The curly brace syntax for sets was introduced in Python 2.7, and so for
compatibility with 2.6 we need to avoid using it for now.

python/google/protobuf/internal/descriptor_pool_test.py

@@ -269,7 +269,7 @@ def testFindAllExtensions(self):
     self.pool.AddExtensionDescriptor(another_field)
 
     extensions = self.pool.FindAllExtensions(factory1_message)
-    expected_extension_numbers = {one_more_field, another_field}
+    expected_extension_numbers = set([one_more_field, another_field])
     self.assertEqual(expected_extension_numbers, set(extensions))
     # Verify that mutating the returned list does not affect the pool.
     extensions.append('unexpected_element')