Add BytesWriter

[bschoenmaeckers]

Testing if we can add support for this before python 3.15.

closes #4003

[bschoenmaeckers]

It’s a shame that _PyBytes_Resize is not available on the limited api. 😢

[davidhewitt]

I wonder if it's viable to just use a Vec<u8> until calling .finish(). This might defeat the point of the API for users, but at the same time it would let us support it cleanly on all versions 🤔

[bschoenmaeckers]

I wonder if it's viable to just use a Vec<u8> until calling .finish(). This might defeat the point of the API for users, but at the same time it would let us support it cleanly on all versions 🤔

I was hesitant to fallback to Vec for the same reason. But it seems fine to do it like this to make it work.

[bschoenmaeckers]

How can I make pyo3-ffi-check happy? It is complaining that PyBytesWriter does not exist on pre-1.15. This is expected but I cannot let it point to the existing _PyBytesWriter because the layout is different.

 error[E0412]: cannot find type `PyBytesWriter` in module `bindings` --> src\main.rs:70:5 | 70 | pyo3_ffi_check_macro::for_all_structs!(check_struct); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in `bindings`