Fix '__bool__' method (#735)

The method returns the truth when the stack is empty

data_structures/stacks/stack.py

@@ -17,7 +17,7 @@ class Stack(object):
         self.limit = limit
 
     def __bool__(self):
-        return not bool(self.stack)
+        return bool(self.stack)
 
     def __str__(self):
         return str(self.stack)