Let pybind11 modules release the GIL when used with free-threaded Python

[sebp]

While free-threading wheels are being built, the pybind11 modules currently do not release the GIL. Therefore, when running osqp with free-threaded Python, one gets the warning:

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

This pull request releases the GIL, based on https://pybind11.readthedocs.io/en/stable/advanced/misc.html#free-threading-support

Minimum required version for dependencies have been raised to versions that added support for free-threaded Python when using Python 3.14 or later.

[CLAassistant]

All committers have signed the CLA.

[vineetbansal]

@sebp - the osqp C++ bindings for python are not thread-safe though, so I'm fairly certain that we do need to hold the GIL. Do you have evidence to the contrary?

[sebp]

@vineetbansal I do not have evidence that the C++ code is thread-safe. I would consider this PR as an experiment to provide free-threaded wheels so that thread-safety can be evaluated.