input: Write a python function to find the maximum value in a list of numbers.
  assert find_max([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) == 10
  assert find_max([10, 20, 45, 67, 84, 93]) == 93
  assert find_max([5, 7, 9, 8, 6, 4, 3]) == 9
output: def find_max(numbers):
      if not numbers:
          return None
      max_value = numbers[0]
      for number in numbers:
          if number > max_value:
              max_value = number
      return max_value