Add PyBytes::new_with_writer

[bschoenmaeckers]

Adds a new constructor on PyBytes that allows creating a python bytes object using a std::io::Write writer. I've exposed the writer as &mut dyn Write dyn object so we do not have to expose the internal writer implementation. We can change this to &mut PyBytesWriter at a later stage if that seems necessary.

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`