Fix some easy type errors (#1502)

pwndbg/heap/ptmalloc.py

@@ -5,6 +5,7 @@ from enum import Enum
 from typing import Any
 from typing import Dict
 from typing import Set
+from typing import Union
 
 import gdb
 

pwndbg/lib/memoize.py

@@ -10,6 +10,7 @@ from collections.abc import Hashable
 from typing import Any
 from typing import Callable
 from typing import Dict
+from typing import List
 
 debug = False