Adapt to changed function name in Python 3.13 (#4902)

According to https://docs.python.org/3.13/whatsnew/3.13.html:

Add PyThreadState_GetUnchecked() function: similar to
PyThreadState_Get(), but don't kill the process with a fatal error if
it is NULL. The caller is responsible to check if the result is
NULL. Previously, the function was private and known as
_PyThreadState_UncheckedGet().

include/pybind11/detail/type_caster_base.h

@@ -486,8 +486,10 @@ PYBIND11_NOINLINE handle get_object_handle(const void *ptr, const detail::type_i
 inline PyThreadState *get_thread_state_unchecked() {
 #if defined(PYPY_VERSION)
     return PyThreadState_GET();
-#else
+#elif PY_VERSION_HEX < 0x030D0000
     return _PyThreadState_UncheckedGet();
+#else
+    return PyThreadState_GetUnchecked();
 #endif
 }