return dict copy on sel.attrib

This is done to prevent:

- changes in this object be reflected in a tree;
- keeping a reference to a tree and preventing response GC;
- isinstance(sel.attrib, dict) to be False

Thanks @kmike !

parsel/selector.py

@@ -338,7 +338,7 @@ def remove_namespaces(self):
     def attrib(self):
         """Return the attributes dictionary for underlying element.
         """
-        return self.root.attrib
+        return dict(self.root.attrib)
 
     def __bool__(self):
         """