Python 3.14 with marshal::dumps on slices: unmarshallable object

Bug Description

Python 3.14 adds marshal version 5 with support for slices. PyO3 continues to use version 4 with the consequence that simple code with slices like a[:] can no longer be marshalled in Python 3.14.

For example, this

let compiled = PyCode::compile(
    py,
    CString::new("a[:]").unwrap().as_c_str(),
    CString::new("foo.py").unwrap().as_c_str(),
    PyCodeInput::File,
)?;
pyo3::marshal::dumps(&compiled, pyo3::marshal::VERSION)?;

will fail with

E       ValueError: unmarshallable object

It works in Python 3.13 and earlier; it also works with VERSION = 5.

Steps to Reproduce

See above.

Backtrace

Your operating system and version

Linux 6.12.53-1-lts

Your Python version (python --version)

3.14

Your Rust version (rustc --version)

rustc 1.90.0 (1159e78c4 2025-09-14) (Arch Linux rust 1:1.90.0-4)

Your PyO3 version

0.27.9

How did you install python? Did you use a virtualenv?

uv venv --python 3.14

Additional Info

No response

No repository file or test is named. Start by reproducing the shown PyCode::compile and pyo3::marshal::dumps example with Python 3.14, then inspect how marshal VERSION is selected; done means slices marshal successfully on Python 3.14 while the supported older versions remain working.