Cannot interrupt top-level coroutine

It looks like interrupting a cell execution which consists of awaiting a top-level coroutine, e.g.:
```python
import asyncio

async def main():
await asyncio.sleep(5)

await main()
```
doesn't take effect immediately, but once the coroutine completes execution.

See #880 for a test that reveals this bug.

从 issue 中顶层 coroutine 的复现代码开始，并检查 #880 引用的测试。验证等待 asyncio.sleep(5) 时的中断处理行为；完成的标准是，cell 被中断后会立即停止，而不是等待 coroutine 完成。

由索引模型根据 Issue 内容生成。