Add support for serde-json arbitrary_precision

[jpmckinney]

When the arbitrary_precision feature is enabled, pythonize serializes all numbers like {'$serde_json::private::Number': '2'} instead of 2.

To reproduce

Cargo.toml

[package]
name = "pythonize_bug"
version = "0.1.0"
edition = "2024"

[lib]
crate-type = ["cdylib"]

[dependencies]
pyo3 = { version = "0.27", features = ["extension-module"] }
pythonize = "0.27"
serde_json = { version = "1.0", features = ["arbitrary_precision"] }

pyproject.toml

[build-system]
requires = ["maturin>=1.4,<2.0"]
build-backend = "maturin"

[project]
name = "pythonize_bug"
version = "0.1.0"

[tool.maturin]
module-name = "pythonize_bug"

src/lib.rs

use pyo3::prelude::*;
use pythonize::{depythonize, pythonize};
use serde_json::Value;

#[pyfunction]
fn test_roundtrip<'py>(py: Python<'py>, obj: Bound<'py, PyAny>) -> PyResult<Bound<'py, PyAny>> {
    let value: Value = depythonize(&obj)?;
    Ok(pythonize(py, &value)?)
}

#[pymodule]
fn pythonize_bug(m: &Bound<'_, PyModule>) -> PyResult<()> {
    m.add_function(wrap_pyfunction!(test_roundtrip, m)?)?;
    Ok(())
}

Install

uv venv
uv pip install maturin
uv run maturin develop

Run

$ uv run python -c 'from pythonize_bug import test_roundtrip; print(test_roundtrip(18446744073709551615))'
{'$serde_json::private::Number': '18446744073709551615'}

If , features = ["arbitrary_precision"] is removed from Cargo.toml and uv run maturin develop is run:

$ uv run python -c 'from pythonize_bug import test_roundtrip; print(test_roundtrip(18446744073709551615))'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
Exception: JSON number out of range

[jpmckinney]

Related: serde-rs/json#896 serde-rs/json#559