input: Write a python function to calculate the average of a list of numbers.
  assert avg([1, 5, 8, 10]) == 6.0
  assert avg([0, 2, 4, 6, 8]) == 4.0
  assert avg([1, 1, 1, 1, 1]) == 1.0
output: def avg(numbers):
      return sum(numbers) / len(numbers)