Add missing type annotation for previous_last_names

Author: ike-agu

prep_exercise/9_inheritance.py

@@ -12,7 +12,7 @@ def get_name(self) -> str:
 class Child(Parent):
     def __init__(self, first_name: str, last_name: str):
         super().__init__(first_name, last_name)
-        self.previous_last_names = []
+        self.previous_last_names: list[str] = []
 
     def change_last_name(self, last_name) -> None:
         self.previous_last_names.append(self.last_name)