input: Write a python function to calculate the standard deviation of a list of numbers.
  assert calculate_std([1, 2, 3, 4, 5]) == 1.4142135623730951
  assert calculate_std([10, 20, 30, 40, 50]) == 14.142135623730951
  assert calculate_std([2, 4, 4, 4, 5, 5, 7, 9]) == 2.0
output: def calculate_std(numbers):
      mean = sum(numbers) / len(numbers)
      variance = sum((x - mean) ** 2 for x in numbers) / len(numbers)
      std_dev = variance ** 0.5
      return std_dev