Add arcstr::ArcStr and arcstr::Substr python convertions

[zao111222333]

arcstr is a popular crate which provides a efficient reference-counted string type (a better Arc<str>).
This PR adds arcstr::ArcStr and arcstr::Substr convertions from/to python str type.

[zao111222333]

Hi everyone, I have no idea why nox -s clippy-all failed in CI / clippy/x86_64-unknown-linux-gnu/beta
see https://github.com/PyO3/pyo3/actions/runs/12721445851/job/35464394579?pr=4852
while it works in CI / clippy/x86_64-unknown-linux-gnu/stable
Is it ok/negligible?

[davidhewitt]

Thanks for the PR. I am hesitant to add this. We have to draw the line somewhere on what we can accept for both maintenance burden and the limits of CI resources, we've rejected proposals before for similar reasons. arcstr is about 10x less downloaded than any of our existing optional features.

If there's a clear demand for this, I can be persuaded. If it's just a few users, then I think this conversion is not particularly difficult for users to implement in their own code.

[zao111222333]

Thanks for the PR. I am hesitant to add this. We have to draw the line somewhere on what we can accept for both maintenance burden and the limits of CI resources, we've rejected proposals before for similar reasons. arcstr is about 10x less downloaded than any of our existing optional features.

If there's a clear demand for this, I can be persuaded. If it's just a few users, then I think this conversion is not particularly difficult for users to implement in their own code.

Thank you for your reply. I used to think that's a popular crate, but the downloaded statistic did seem to be not so much. In this case it likely I have to make a wrapper to implement those conversion traits.
And if you agree that adopt this feature, I'd like to maintain that :)