input: Write a python function to calculate the mean of a list of numbers.
  assert calculate_mean([1, 2, 3, 4, 5]) == 3.0
  assert calculate_mean([10, 20, 30, 40, 50]) == 30.0
  assert calculate_mean([7, 14, 21, 28]) == 17.5
output: def calculate_mean(numbers):
      return sum(numbers) / len(numbers)