Consider support for free-threaded Python builds (f.e. 3.14t)

nanobind supports it with:
```cmake
nanobind_add_module(_PyPartMC FREE_THREADED STABLE_ABI ${PyPartMC_sources})
```
We skip the builds in `buildwheels.yml`:
```yml
CIBW_SKIP: "*-win32 *-manylinux_i686 pp* *musllinux* cp3??t-*"
```