asyncio updates for Python 3.14

There have been a significant API changes in asyncio with deprecations and removals in 3.14

The following APIs are deprecated:

• asyncio.iscoroutinefunction
• asyncio.AbstractEventLoopPolicy
• asyncio.DefaultEventLoopPolicy
• asyncio.WindowsSelectorEventLoopPolicy
• asyncio.WindowsProactorEventLoopPolicy
• asyncio.get_event_loop_policy()
• asyncio.set_event_loop_policy()
• asyncio.set_event_loop()

The following APIs are removed:

• asyncio.get_child_watcher()
• asyncio.set_child_watcher()
• asyncio.AbstractEventLoopPolicy.get_child_watcher()
• asyncio.AbstractEventLoopPolicy.set_child_watcher()
• asyncio.AbstractChildWatcher
• asyncio.FastChildWatcher
• asyncio.MultiLoopChildWatcher
• asyncio.PidfdChildWatcher
• asyncio.SafeChildWatcher
• asyncio.ThreadedChildWatcher

该 issue 未指定文件或测试；首先定位 typeshed 中的 asyncio 存根，并将其与 Python 3.14 的 asyncio 文档进行比较。更新对所列已弃用和已移除 API 的覆盖范围，然后运行相关的 typeshed 检查；当存根准确反映 Python 3.14 时，即表示完成。