Expand compatibility using qtpy for PyQt6, PyQt5, PySide6, and PySide2

[GenevieveBuckley]

The qtpy abstraction layer allows projects to simultaneously support backends including PyQt6, PyQt5, PySide6, and PySide2.

This is a lot more convenient than users having to make edits like this one #6 to the code if they happen to use a different version of Qt, and it's a lot nicer than asking users to check out a specific commit to use the code - now one install works for all situations.

I've tested this PR locally, and the demo.py script runs well for all four backends. I have another branch where I've added CI tests with github actions and can submit that as a separate PR if you like.

[GenevieveBuckley]

I've added the contents of GitHub's .gitignore template for Python projects, mostly because I almost accidentally committed the __pycache__ folder or other files a couple of times.
It makes sense to me that this wasn't included earlier because this project was originally forked from a C++ project (https://github.com/snowwlex/QtWaitingSpinner), but I think this will make things a lot more convenient.

[z3ntu]

While this looks pretty nice, I'm not really interested in this project anymore (haven't used this code myself since probably 2017). So I'd recommend you maintain this in a fork yourself.

But I'd be happy to add something to the readme to point to your repo (since mine has apparently somehow become semi-popular) for anybody looking for qtpy/pyside (or pyqt5) support.

Does this sound okay?

[GenevieveBuckley]

I'm sorry, I didn't realise this project was inactive (archiving the repo might make this more obvious to casual users).

I'm not sure whether I'm likely to keep the fork. I made these changes while I was testing things out, and it's not clear to me yet how actively I will use it.