input: Write a function to return the maximum number in a list.
  assert find_max([1, 15, 20, 6]) == 20
  assert find_max([3, 8, 2, 10]) == 10
  assert find_max([-5, -2, -1, -10]) == -1
  assert find_max([7]) == 7
  assert find_max([]) == None
output: def find_max(lst):
      if not lst:
          return None
      max_num = lst[0]
      for num in lst:
          if num > max_num:
              max_num = num
      return max_num