fix: support free-threaded CPython with GIL disabled

[colesbury]

Description

Accesses to the internals struct now goes through a with_internals([] { ... }); statement. In the free-threaded build, the internals mutex is locked for the duration of the access.

The registered_instances map has its own locking scheme because it may be frequently accesses by multiple thread. The map is shareded by address and a lock per shard is used to increase concurrency.

Other smaller changes:

• Add gil_not_used() tag to indicate that a module supports running with the GIL disabled. See PyUnstable_Module_SetGIL.
• Use PyMem_MALLOC for tp_doc in 3.13+
• Add dict_getitemstringref to wrap new PyDict_GetItemStringRef API

Suggested changelog entry:

* Support free-threaded CPython (3.13t)
* Add ``py::mod_gil_not_used()`` tag to indicate if a module supports running with the GIL disabled.

[henryiii]

Changelog in the description is a little out of date, too.

[colesbury]

Yes, please apply it