code: def post_presign_signature(date, region, secret_key, policy_str):
      
      signing_key = generate_signing_key(date, region, secret_key)
      signature = hmac.new(signing_key, policy_str.encode('utf-8'),
                           hashlib.sha256).hexdigest()
  
      return signature
docs: Calculates signature version '4' for POST policy string.
  
      :param date: datetime formatted date.
      :param region: region of the bucket for the policy.
      :param secret_key: Amazon S3 secret access key.
      :param policy_str: policy string.
      :return: hexlified sha256 signature digest.