input: Write a python function to find the maximum number in a list.
  assert find_max_number([1, 2, 3, 4, 5]) == 5
  assert find_max_number([-1, -2, -3, -4, -5]) == -1
  assert find_max_number([10, 9, 8, 7, 6]) == 10
output: def find_max_number(lst):
      if not lst:
          return None
      max_number = lst[0]
      for num in lst:
          if num > max_number:
              max_number = num
      return max_number