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