ipykernel should use default `WindowsProactorEventLoopPolicy` rather than `WindowsSelectorEventLoopPolicy` for windows

从 init() 中的 Windows 事件循环策略代码块开始，阅读所引用的 pull request 和 issue，尤其是已回滚的更改和 issue 1190。在更改策略之前确定回归风险，然后验证使用 create_subprocess_exec 的 Windows asyncio 代码能够正常工作，不会出现之前报告的故障。

there were some issues discussed about this problem before https://github.com/ipython/ipykernel/pull/1184,
then https://github.com/ipython/ipykernel/issues/1190 caused https://github.com/ipython/ipykernel/pull/1194 reverted the above PR.
I tried the https://github.com/ipython/ipykernel/issues/1190 but it's unable to reproduce now in ipykernel version 7.1.

the asyncio on windows needs WindowsProactorEventLoopPolicy to have the ability to use create_subprocess_exec, which is used in practice places, for example:

• using playwright which is a new automation tools after selenium.

import asyncio
from playwright.async_api import async_playwright

async def main():
    async with async_playwright() as p:
        browser = await p.chromium.launch()
        page = await browser.new_page()
        await page.goto("https://playwright.dev")
        print(await page.title())
        await browser.close()

await main()
# asyncio.run(main())

         on windows, the above codes is not useable in ipython, junpyter, or jupyter notebook in vscode because of WindowsSelectorEventLoopPolicy lacking of create_subprocess_exec

• a another use case like a issue in this repo https://github.com/ipython/ipykernel/issues/391, this PR using default event loop WindowsProactorEventLoopPolicy can also close this issue.

Even If the above https://github.com/ipython/ipykernel/issues/1190 is still reproduceable, then should fix it, to make asyncio create_subprocess_exec is working on windows in jupyter.

therefore I created a PR again to get rid of using WindowsSelectorEventLoopPolicy on windows, since https://github.com/ipython/ipykernel/releases/tag/v7.0.0 dropped support for python 3.9, so the PR is simply removing the whole block code of the init() that checked python version

ps.
there is a old issue before https://github.com/ipython/ipykernel/pull/1184 which talked about that tornado 6.1 used background threads and which may reduce performance, but result of running test in https://github.com/ipython/ipykernel/pull/1184 showed performance improved rather than reduced. and that asyncio on windows has ability to use a common api create_subprocess_exec should be more important than threads in background problem