Make it compatible with Free Threading (aka no-gil)

[thepabloaguilar]

Since Python 3.14 the no-gil feature is not experimental anymore but optional, when some of the dependencies are not marked that can run safely in those versions Python interpreter fallback to the default mode with GIL. This can impact when running on production but also when running tests:

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

It would be nice if this lib could be compatible with no-gil, if needed I can try to help!

[seddonym]

Thanks for the issue, I would like to support free threading but am not sure what is involved or how much work it is.

Any input on that welcome!