input: Write a python function to calculate the mean of a list of numbers.
  assert mean([1, 2, 3, 4, 5]) == 3.0
  assert mean([10, 20, 30, 40, 50]) == 30.0
  assert mean([5, 7, 9, 3]) == 6.0
output: def mean(numbers):
      return sum(numbers) / len(numbers)