Signature for this request is not valid error for Binance US API

Question

I am getting a signature not valid error while connecting to Binance API endpoints that require authentication. There is a similar query in following link however it is specific to Binance, I guess and this question is specific to Binance US. I tried to use the methods in below link though and it didn't work.

Flutter binance api signature

Following is the Python code

import urllib.parse
import hashlib
import hmac
import base64
import requests

api_url = "https://api.binance.us"

# get binanceus signature
def get_binanceus_signature(data, secret):
    postdata = urllib.parse.urlencode(data)
    message = postdata.encode()
    byte_key = bytes(secret, 'UTF-8')
    mac = hmac.new(byte_key, message, hashlib.sha256).hexdigest()
    return mac

# Attaches auth headers and returns results of a POST request
def binanceus_request(uri_path, data, api_key, api_sec):
    headers = {}
    headers['X-MBX-APIKEY'] = api_key
    signature = get_binanceus_signature(data, api_sec) 
    params={**data, "signature": signature}           
    req = requests.get((api_url + uri_path), params=params, headers=headers)
    return req.text

api_key = "vmPUZE6mv9SD5VNHk4HlWFsOr6aKE2zvsw0MuIgwCIPy6utIco14y7Ju91duEh8A"
secret_key = "NhqPtmdSJYdKjVHjA7PZj4Mge3R5YNiP1e3UZjInClVN65XAbvqqM6A7H5fATj0j"

uri_path = "/api/v3/openOrders"
data = {
    "symbol": "BTCUSDT", 
    "timestamp": 1499827319559
}

get_open_order_result = binanceus_request(uri_path, data, api_key, secret_key)

Here is the full dart code.

class BinanceUSRestClient {
  final String timestamp = DateTime.now().millisecondsSinceEpoch.toString();

  Future<http.Response> getResponse({
    required String secret,
    required String apiKey,
    required String path,
    Map<String, dynamic>? queryParams,
  }) async {
    //Header
    Map<String, String> headers = {};
    headers['X-MBX-APIKEY'] = apiKey;

    //Params
    Map<String, dynamic> params = {};
    if (queryParams != null) {
      params.addAll(queryParams);
    }
    params['signature'] = createSignature(secret, queryParams);
    params['timestamp'] = timestamp;

    final Uri uri = Uri.https('api.binance.us', path, params);
    http.Response response = await http.get(
      uri,
      headers: headers,
    );
    return response;
  }

  String createSignature(String secret, Map<String, dynamic>? data) {
    final String jsonString = jsonEncode(data);
    final List<int> message = utf8.encode(jsonString);
    final List<int> key = utf8.encode(secret);
    final List<int> mac = Hmac(sha256, key).convert(message).bytes;
    final String signature = hex.encode(mac);
    return signature;
  }
}

If it helps, here is the API documentation. Could someone help me in resolving the error?