Add type hints to pancake_sort (#14737)

sorts/pancake_sort.py

@@ -8,8 +8,10 @@
 python pancake_sort.py
 """
 
+from collections.abc import MutableSequence
 
-def pancake_sort(arr):
+
+def pancake_sort(arr: MutableSequence[int]) -> MutableSequence[int]:
     """Sort Array with Pancake Sort.
     :param arr: Collection containing comparable items
     :return: Collection ordered in ascending order of items