Make pysorteddict thread-safe

[tfpf]

This issue will serve as discovery to record what I need to change and what I don't need to to support the free-threaded build of Python 3.14.

• import_supported_key_types need not lock anything

CPython holds a per-module lock during import. […] module initialization happens exactly once per interpreter in one C thread. […] you might set up a global static cache that is read-only after module initialization like this:

static int *cache = NULL;

PyMODINIT_FUNC
PyInit__module(void)
{
    PyObject *mod = PyModule_Create(&module);
    if (mod == NULL) {
        return NULL;
    }

    // don't need to lock or do anything special
    cache = setup_cache();

    // do rest of initialization
}

You can then read from cache at runtime in a context where you know the module is initialized without worrying about whether or not the per-module static cache is initialized.

Although the example is for single-phase initialisation, since initialisation is done only once, it is safe to conclude that static global read-only variables need not be locked before initialisation even when using multi-phase initialisation.

Updating Extension Modules - Python Free-Threading Guide

• SortedDict::key_type should be initialised using a mutex lock before and after which it should be compared with nullptr

The first check serves as a way to avoid locking the mutex if the pointer is initialised. The second check serves to avoid setting the pointer to some other value if another initialised it already.