[python] `uv sync` fails using a Python 3.14 interpreter

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### Description

To reproduce, make sure the `requires-python` in `pyproject.toml` supports 3.14, and make sure you are using python e.g. 3.14.2 with UV (>= 3.14 seems to be needed), and then run:
```sh
uv venv
source .venv/bin/activate
uv sync
```
This then failed for me with the following (shortened) log:
```
Resolved 100 packages in 0.53ms
× Failed to build `libcst==1.7.0`
├─▶ The build backend returned an error
╰─▶ Call to `setuptools.build_meta:__legacy__.build_wheel` failed (exit status: 1)

[stdout]
running bdist_wheel
running build
running build_py

...

--- stderr
error: the configured Python interpreter version (3.14) is newer than PyO3's maximum supported version (3.13)
= help: please check if an updated version of PyO3 is available. Current version: 0.23.5
= help: set PYO3_USE_ABI3_FORWARD_COMPATIBILITY=1 to suppress this check and build anyway using the stable ABI
warning: build failed, waiting for other jobs to finish...
error: `cargo rustc --lib --message-format=json-render-diagnostics --manifest-path native/libcst/Cargo.toml --release -v --features pyo3/extension-module --crate-type cdylib --` failed with code 101

hint: This usually indicates a problem with the package or the build environment.
help: `libcst` (v1.7.0) was included because `opentitan` (v0.0.0) depends on `libcst`
```

Start by reproducing the failure with the stated Python 3.14, uv venv, and uv sync commands. Then inspect pyproject.toml and the libcst build referenced through native/libcst/Cargo.toml, including the PyO3 0.23.5 error; done means uv sync completes successfully on the supported Python 3.14 setup.

Written by the indexing model from the issue text.