refact: add typing to decorator

Author: boriel

src/api/decorator.py

@@ -1,13 +1,15 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
 
+from typing import Callable
+
 
 class classproperty:
     """ Decorator for class properties.
     Use @classproperty instead of @property to add properties
     to the class object.
     """
-    def __init__(self, fget):
+    def __init__(self, fget: Callable):
         self.fget = fget
 
     def __get__(self, owner_self, owner_cls):