Add support for serde-json arbitrary_precision

When the `arbitrary_precision` feature is enabled, pythonize serializes all numbers like `{'$serde_json::private::Number': '2'}` instead of `2`. ## To reproduce ### Cargo.toml ```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 ```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 ```rust 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 ```bash uv venv uv pip install maturin uv run maturin develop ``` ### Run ```shell-session $ 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: ```shell-session $ 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 ```