input: Write a python function to find the largest number in a list.
  assert largest_number([1, 10, 5, 6, 9, 8, 20]) == 20
  assert largest_number([4, 2, 8, 6, 3]) == 8
  assert largest_number([10, 20, 30, 40, 50]) == 50
output: def largest_number(lst):
      return max(lst)