Fix being able to call arg-less #[new] with any args from Python

[birkenfeld]

Fixes #2748

Note that the removed check only triggered for #[new] anyway, since other arg-less pymethods use the NOARGS argument convention which is not available for tp_new.

There's some confusion going on with the py argument, I'll open an issue for that.

[birkenfeld]

Oh great, another MSRV bump (rayon-core) in a semver-compatible version bump. To the 2021 edition nonetheless.

[davidhewitt]

Ooof, thanks for fixing!

[birkenfeld]

Hmm, that test failure is interesting. The base class' __new__ is called implicitly, it seems, with the same arguments?

[birkenfeld]

Added a bit to the guide that explains the behavior.

[davidhewitt]

Thanks, this is definitely a correct fix and it's good to document existing behaviour.

I think there's potential for improvement around how to control invocation of __new__ and __init__ with inheritance, however I don't have a good answer for that right now and it's definitely for a future discussion anyway.