Detach removed node in pop_tail for consistency with remove()

Author: PratiAmalden

Sprint-2/implement_linked_list/linked_list.py

@@ -27,15 +27,20 @@ def pop_tail(self): # remove element from end of list
         if not self.tail:
             return None
 
-        value = self.tail.value # 2
-        new_tail = self.tail.previous # 1
+        old_tail = self.tail
+        value = self.tail.value
+        new_tail = self.tail.previous
 
         if new_tail:
             new_tail.next = None
         else:
             self.head = None
 
         self.tail = new_tail
+
+        old_tail.previous = None
+        old_tail.next = None
+
         return value
 
 # [4, 1, 2] - 1