gh-108867: Add PyThreadState_GetUnchecked() function (#108870)

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

Python/pystate.c

@@ -1908,7 +1908,7 @@ PyThreadState_SetAsyncExc(unsigned long id, PyObject *exc)
 //---------------------------------
 
 PyThreadState *
-_PyThreadState_UncheckedGet(void)
+PyThreadState_GetUnchecked(void)
 {
     return current_fast_get(&_PyRuntime);
 }