input: Write a function to find the maximum number in a list of integers.
  assert find_max([3, 5, 12, 20, 6, 4]) == 20
  assert find_max([-1, -5, -3, -4]) == -1
  assert find_max([1, 2, 3, 4, 5]) == 5
output: def find_max(numbers):
      if not numbers:
          return None
      max_number = numbers[0]
      for num in numbers:
          if num > max_number:
              max_number = num
      return max_number