input: Write a python function to calculate the mean of a list of numbers.
  assert mean([60, 54, 77, 78, 37, 35]) == 56.833333333333336
  assert mean([10, 20, 30, 40, 50]) == 30
  assert mean([100, 200, 300]) == 200
output: def mean(numbers):
      return sum(numbers) / len(numbers)