Support GIL free python

Related to https://github.com/conda-forge/spglib-feedstock/pull/122, @LecrisUT, is the following treatment useful?

```c
#include <pybind11/pybind11.h>
namespace py = pybind11;

PYBIND11_MODULE(example, m, py::mod_gil_not_used()) {
    ...
}
```

- https://docs.python.org/3/howto/free-threading-extensions.html
- https://py-free-threading.github.io/porting-extensions/