ENH: stats: enable free-threading support for Cython extensions #20267

What is the problem this feature will solve?

Adding the module-level directive # cython: freethreading_compatible = True to astropy.stats._stats (assuming it is thread-safe) allows Astropy to run on free-threaded Python builds without re-enabling the GIL.

Currently, when running tests on a free-threaded Python build (verified on Python 3.15t), importing this module causes pytest to emit the following warning:

RuntimeWarning: The global interpreter lock (GIL) has been enabled to load module 'astropy.stats._stats', which has not declared that it can run safely without the GIL. To override this behavior and keep the GIL disabled (at your own risk), run with PYTHON_GIL=0 or -Xgil=0.

Describe the desired outcome

Add # cython: freethreading_compatible = True to astropy.stats._stats (and potentially other Cython modules if applicable) so that the module can be imported in free-threaded Python without automatically re-enabling the GIL.

Additional context

No response