您的连接字符串错误。请仔细检查是否正确，并且调用服务器的IP已列入白名单。

我遇到了同样的问题，下面的代码适用于python3.9.1

import pymongo
import urllib 
MONGODB_USERNAME = urllib.parse.quote_plus('test')
MONGODB_PASSWORD = urllib.parse.quote_plus('tset@000')
MONGODB_DATABASE = 'sampledb'
MONGODB_URL = "mongodb://"+MONGODB_USERNAME+":"+MONGODB_PASSWORD+"@cluster0.wq5js.mongodb.net/"+MONGODB_DATABASE+"?retryWrites=true&w=majority"
client = pymongo.MongoClient(MONGODB_URL)

下面的Python代码适用于Mongodb Atlas。用Python 3.8和3.9测试。
参考：https://www.mongodb.com/docs/atlas/troubleshoot-connection/#connection-string-issues

from pymongo import MongoClient
from urllib.parse import quote_plus

USERNAME = quote_plus('test')
PASSWORD = quote_plus('test@12345')
DATABASE = 'sampledb'
URI = "mongodb+srv://" + USERNAME + ":" + PASSWORD + "@" + DATABASE_HOST

client = MongoClient(URI)
conn = client[DATABASE]
conn.command('ping')

请注意，Mongodb Atlas不支持用户管理。不支持的命令列表提到here。