input: Write a python function to generate a randomness score based on a given string.
  assert randomness_score("hello") == randomness_score("hello")
  assert randomness_score("world") != randomness_score("hello")
  assert isinstance(randomness_score("test"), float)
output: import hashlib
  
  def randomness_score(s: str) -> float:
      hash_obj = hashlib.sha256(s.encode())
      hex_dig = hash_obj.hexdigest()
      return int(hex_dig, 16) / (1 << 256)