input: Write a function to calculate the standard deviation of a list.
  assert calculate_sd([1, 2, 3, 4, 5]) == 1.4142135623730951
  assert calculate_sd([10, 12, 23, 23, 16, 23, 21, 16]) == 4.898979485566356
  assert calculate_sd([100, 100, 100, 100]) == 0.0
output: def calculate_sd(data):
      if len(data) == 0:
          return 0.0
  
      mean = sum(data) / len(data)
      variance = sum((x - mean) ** 2 for x in data) / len(data)
      return variance ** 0.5