Support the free threaded build

Currently, mlx and any package that depends on mlx fails to install on the free-threaded build:

```
goldbaum at Nathans-MBP in ~/Documents/guidance on implicit-event-loop
± python -VV
Python 3.14.0 free-threading build (main, Oct  8 2025, 10:32:29) [Clang 17.0.0 (clang-1700.3.19.1)]

goldbaum at Nathans-MBP in ~/Documents/guidance on implicit-event-loop
± pip install mlx
ERROR: Could not find a version that satisfies the requirement mlx (from versions: none)

[notice] A new release of pip is available: 25.2 -> 25.3
[notice] To update, run: pip install --upgrade pip
ERROR: No matching distribution found for mlx
```

And indeed, there aren't any cp314t wheels on PyPI.

It'd be really nice to be able to use mlx on the free-threaded build. Both PyTorch and jaxlib ship free-threaded wheels.

Also see https://py-free-threading.github.io for more about using the free-threaded build and https://py-free-threading.github.io/porting-extensions/ for more about updating extensions.

Myself and my team at Quansight have been actively working on adding support for the free-threaded build in community packages for the past year and a half. I can also loop in CPython core devs and other experts who can offer advice.

Currently our short-term goal is to get vLLM running on the free-threaded build. This is in support of that, since mlx is an indirect dependency on apple silicon macs, via xgrammar. See https://github.com/vllm-project/vllm/issues/28762.

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