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

[AlexJones0]

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:

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`