PyQt6 stubs example with QtCore

[TilmanK]

This is a follow-up PR for the discussion about PyQt6 here: #69

This PR contains some adjustments to the lib, but most importantly the adapted pyi-File for QtCore. It's basically a merge between the upstream pyi-File and the current stubs (or more precisely, the fixes of the current stubs).

Here are some things that must be discussed:

• To easily compare both files (current QtCore.pyi and upstream pyi) I ran with Black over both of them. The result is that there are a lot of differences between the current and the upstream pyi file. Shall I revert the changes by Black? Shouldn't be that much work.
• The original stubs as well as "our" stubs aren't very clear what to do with bytes and QByteArray. Actually, the stubs define a Buffer (PyQt6.sip.Buffer) like this: Buffer = Union[bytes, bytearray, memoryview, array, voidptr], but don't use it a lot. What we use is typing.Union[QByteArray, bytes, bytearray]. The stubs use either a QByteArray, or typing.Union[QByteArray, bytes, bytearray, memoryview]. We should be consistent...
• What's the point in using the type variable in QtCore.pyi:55 QObjectT?

I'm not sure if this is the right way to go forward with PyQt6 (@The-Compiler already said correctly that we could or should use some automation) - any feedback is highly appreciated. And if someone has a better way or is more experienced - go on and take over. I will just need the stubs in the end ;)