input: Write a python function to calculate the mean of a list of numbers.
  assert mean([1, 5, 8, 9]) == 5.75
  assert mean([10, 20, 30, 40]) == 25.0
  assert mean([2, 4, 6, 8, 10]) == 6.0
output: def mean(numbers):
      return sum(numbers) / len(numbers)