input: Write a python function to calculate the average of a list of numbers.
  assert average([1, 4, 7, 3]) == 3.75
  assert average([10, 20, 30, 40]) == 25.0
  assert average([5, 5, 5, 5, 5]) == 5.0
output: def average(numbers):
      return sum(numbers) / len(numbers)