input: Write a python function to find the maximum number in a list.
  assert max_number([5, 6, 7, -2, 3]) == 7
  assert max_number([1, 2, 3, 4, 5]) == 5
  assert max_number([-10, -20, -30, -5]) == -5
output: def max_number(lst):
      return max(lst)