Added Python 3 compatibility to sample project.

Removed utils file and moved map_path function to settings.py.
Refactored tests for Python 2 and 3 compatibility.
Added inheritance check to get proper determination if a SortableForeignKey field is defined but the specified model does not inherit from Sortable.

adminsortable/utils.py

@@ -1,9 +1,13 @@
 from .models import Sortable, SortableForeignKey
 
 
+def check_inheritance(obj):
+    return issubclass(type(obj), Sortable)
+
+
 def get_is_sortable(objects):
     if objects:
-        if issubclass(type(objects[0]), Sortable):
+        if check_inheritance(objects[0]):
             if objects.count() > 1:
                 return True
     return False
@@ -11,7 +15,8 @@ def get_is_sortable(objects):
 
 def is_self_referential(cls):
     cls_type = type(cls)
-    sortable_subclass = issubclass(cls_type, Sortable)
+    sortable_subclass = check_inheritance(cls_type)
+    # sortable_subclass = issubclass(cls_type, Sortable)
     sortable_foreign_key_subclass = issubclass(cls_type, SortableForeignKey)
     if sortable_foreign_key_subclass and not sortable_subclass:
         return True
@@ -20,11 +25,12 @@ def is_self_referential(cls):
 
 def check_model_is_sortable(cls):
     if cls:
-        if is_self_referential(cls):
-            objects = cls.model.objects
-        else:
-            objects = cls.objects
+        if check_inheritance(cls):
+            if is_self_referential(cls):
+                objects = cls.model.objects
+            else:
+                objects = cls.objects
 
-        if objects.count() > 1:
-            return True
+            if objects.count() > 1:
+                return True
     return False