Fix GIL warning in free-threaded Python

[Copilot]

The apyds._ds module triggers a RuntimeWarning in free-threaded Python (PEP 703) because it doesn't declare GIL safety.

RuntimeWarning: The global interpreter lock (GIL) has been enabled to load module 'apyds._ds', which has not declared that it can run safely without the GIL.

Changes

• Add py::mod_gil_not_used() to the PYBIND11_MODULE declaration in apyds/ds.cc

PYBIND11_MODULE(_ds, m, py::mod_gil_not_used()) {

This is the standard pybind11 approach (2.13+) for declaring free-threading compatibility.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fix warning in free threading python.</issue_title>
<issue_description>```
:491
:491: RuntimeWarning: The global interpreter lock (GIL) has been enabled to load module 'apyds._ds', 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.

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Fix warning in free threading python. #48

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