input: Write a python function to calculate the standard deviation of a list of numbers.
  assert abs(standardDeviation([1, 2, 3, 4, 5]) - 1.5811) < 0.0001
  assert abs(standardDeviation([10, 12, 23, 23, 16, 23, 21, 16]) - 5.2372) < 0.0001
  assert abs(standardDeviation([2, 4, 4, 4, 5, 5, 7, 9]) - 2.0) < 0.0001
output: def standardDeviation(nums):
      mean = sum(nums) / len(nums)
      variance = sum((x - mean) ** 2 for x in nums) / len(nums)
      return variance ** 0.5