input: Write a function to return the maximum number in the provided list. If the list is empty, return None.
  assert find_max([1, 15, 20, 6])==20
  assert find_max([1, 5, -10, 2])==5
  assert find_max([9, 9, 9, 9])==9
  assert find_max([-1, -2, -3, -4])==-1
  assert find_max([])==None
output: def find_max(lst):
      if not lst:
          return None
      return max(lst)