bpo-32604: Fix memory leaks in the new _xxsubinterpreters module. (GH-5507)

(cherry picked from commit 4e9da0d163)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>

Python/pystate.c

@@ -1242,6 +1242,7 @@ _PyCrossInterpreterData_Lookup(PyObject *obj)
             break;
         }
     }
+    Py_DECREF(cls);
     PyThread_release_lock(_PyRuntime.xidregistry.mutex);
     return getdata;
 }