fix building supportedsites.md on Python 3.4 and 3.5

dicts didn't maintain insertion order back then

scripts/supportedsites.py

@@ -363,6 +363,8 @@ def build_extractor_list():
                             if extr.from_url(url).category == category:
                                 root = url[:url.index("/", 8)]
                                 break
+                        else:
+                            continue
                     domains[category] = root + "/"
 
     # sort subcategory lists