Improve examples and README

[temeddix]

This PR improves the example from various aspects.

• Organize imports and variable names with the Ruff formatter.
• Fix aiohttp.ClientSession() error saying that it should be run in an async function.
• Upgrade to PySide6 and PyQt6.
• Use asyncio.Event instead of CancelledError exception to close the app gracefully.
• Use PySide6.QApplication. This is important because this class allows various font, theme, behavior setup. With QApplication.instance, app customization is not possible.
• Remove unnecessary details

And of course, I confirmed that this code works properly.

[temeddix]

Hi @hosaka , I improved examples, README and things that you have mentioned in #96.

Could you review this PR and merge it if it's okay, before publishing the new version?

[hosaka]

Yeah I'm working on it. I have a QML example that I've been meaning to add as well, so I'll update this PR when I put it together.

[hosaka]

I've removed a sentence from the readme about GIL and irrelevant Flutter, JavaScript part. I'm assuming that people who want to use qasync have some basic understanding of what async/coroutines are. Let me know if you're happy with the current state of this PR @temeddix

[temeddix]

It looks great! I only have one question.

Wouldn't this sentence help people get confident about choosing async over threads? Because for beginners, 'thread' might sound faster than 'async', though it actually isn't.

Single-threaded concurrency does not inherently make the app slower, but only safer, because of Python's GIL.

But I am happy with the current status as well :)

[hosaka]

While that sentence does sound encouraging, the readme should describe what the project is and how it should be used to it's full potential, rather than explain abstract concepts :)