ipykernel should use default WindowsProactorEventLoopPolicy rather than WindowsSelectorEventLoopPolicy for windows #1468

there were some issues discussed about this problem before #1184,
then #1190 caused #1194 reverted the above PR.
I tried the #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 async subprocess with live output (on Windows requires ProactorEventLoop which ipykernel doesn't use) #391, this PR using default event loop WindowsProactorEventLoopPolicy can also close this issue.

Even If the above #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 #1184 which talked about that tornado 6.1 used background threads and which may reduce performance, but result of running test in #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