Simplify interp_look_up_id() (python#134257)

Don't use PyInterpreterState_GetID() but get directly the interpreter
'id' member which cannot fail.

Author: vstinner

Python/pystate.c

@@ -1393,10 +1393,8 @@ interp_look_up_id(_PyRuntimeState *runtime, int64_t requested_id)
 {
     PyInterpreterState *interp = runtime->interpreters.head;
     while (interp != NULL) {
-        int64_t id = PyInterpreterState_GetID(interp);
-        if (id < 0) {
-            return NULL;
-        }
+        int64_t id = interp->id;
+        assert(id >= 0);
         if (requested_id == id) {
             return interp;
         }