[FIX] Replace statsmodels.robust.scale.mad with scipy.stats.median_abs_deviation to avoid enabling the GIL

[pvlov]

Hi! Since this is a small diff and I couldn't find a CONTRIBUTING.md, I went ahead and opened a PR. I hope you don't mind!

I wanted to use meegkit in a project that uses a free-threaded Python interpreter, but I ran into the following warning:

<frozen importlib._bootstrap>:491: RuntimeWarning: 
The global interpreter lock (GIL) has been enabled to load module 'statsmodels.robust._qn', 
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.

You can replicate this warning (using uv) with this command:

uv run --no-project --python 3.14t --with statsmodels python -c "import statsmodels.robust.scale"

I checked where/how statsmodels is used and found only one occurrence: statsmodels.robust.scale.mad. This PR replaces it with the equivalent scipy.stats.median_abs_deviation. This makes the package usable in free-threaded contexts while also dropping the statsmodels dependency entirely.

Additionally, I added a test using the built-in sys._is_gil_enabled check to confirm the GIL stays disabled, and added 3.14 and 3.14t to the CI python matrix. Let me know if you want to keep this or if I should drop this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.28%. Comparing base (6b6c1d7) to head (4012864).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
- Coverage   83.29%   83.28%   -0.01%     
==========================================
  Files          25       25              
  Lines        2861     2860       -1     
==========================================
- Hits         2383     2382       -1     
  Misses        478      478              

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