make python 2 3 compatible

Author: tim-vk

dockerfile_parse/parser.py

@@ -359,7 +359,7 @@ def parent_images(self):
                 in_stage = True
                 image, _ = image_from(instr['value'])
                 if image is not None:
-                    image = WordSplitter(b2u(image.to_str()), args=top_args).dequote()
+                    image = WordSplitter(image.to_str(), args=top_args).dequote()
                     parents.append(image)
         return parents