input: Write a function to find the oldest person in a list of tuples with their names and ages.
  assert oldest_person([('John', 18), ('Sarah', 19), ('Bob', 17)])==('Sarah', 19)
  assert oldest_person([('Tom', 21), ('Jerry', 20), ('Spike', 22)])==('Spike', 22)
  assert oldest_person([('Alice', 30), ('Bob', 25), ('Charlie', 35)])==('Charlie', 35)
output: def oldest_person(people):
      return max(people, key=lambda person: person[1])