input: Write a function to calculate the hypotenuse of a right triangle given its side lengths.
  assert hypotenuse_length(3, 4) == 5.0
  assert hypotenuse_length(5, 12) == 13.0
  assert hypotenuse_length(8, 15) == 17.0
output: def hypotenuse_length(a, b):
      return (a**2 + b**2) ** 0.5