bpo-1635741: Explict GC collect after PyInterpreterState_Clear() (GH-21902)

Fix a reference cycle by triggering an explicit GC collection
after calling PyInterpreterState_Clear().

Python/pylifecycle.c

@@ -1286,10 +1286,8 @@ finalize_interp_clear(PyThreadState *tstate)
     /* Clear interpreter state and all thread states */
     PyInterpreterState_Clear(tstate->interp);
 
-    /* Trigger a GC collection on subinterpreters*/
-    if (!is_main_interp) {
+    /* Last explicit GC collection */
     _PyGC_CollectNoFail();
-    }
 
     /* Clear all loghooks */
     /* Both _PySys_Audit function and users still need PyObject, such as tuple.