Only use functions in the limited API

[vyasr]

Description

This PR removes usage of the only method in rmm's Cython that is not part of the Python limited API. Contributes to rapidsai/build-planning#42

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[vyasr]

In addition to base rmm testing, compilation under the limited API is tested in rapidsai/devcontainers#278. Note that CI failed when using rmm 24.06, then successfully compiles using this branch.

[jakirkham]

Thanks Vyas! 🙏

The original idea of PyBytes_AS_STRING was to indirectly do struct member access (skipping type checking that PyBytes_AsString does), which should be fast

If we want to drop PyBytes_AS_STRING for limited API support (which is reasonable), maybe we should drop the function call altogether and let Cython do this for us. This would improve readability and should do the same thing under-the-hood

[vyasr]

The original idea of PyBytes_AS_STRING was to indirectly do struct member access (skipping type checking that PyBytes_AsString does), which should be fast

Right, but the more relevant question I think is whether that performance difference is even visible or if it's lost in all the other noise of rmm (let alone the noise of other RAPIDS packages that use rmm). I ran some quick tests doing something like a few thousand calls to tobytes and I couldn't detect any noticeable performance change before and after this PR.

[jakirkham]

Agreed. That was just historical context. Am ok dropping it. For readability alone it seems worth it

[jakirkham]

/merge

[jakirkham]

Thanks Vyas! 🙏