input: Write a function to find the largest number in a list.
  assert largest([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) == 10
  assert largest([10, 20, 5, 7, 90, 55]) == 90
  assert largest([5, 3, 7, 1, 9, 2]) == 9
output: def largest(numbers):
      return max(numbers)