Remove the reference to the future queue on shutdown

It has no impact on the tests here, and without this change the pyiron_workflow tests hang

pympipool/shared/executorbase.py

@@ -57,6 +57,7 @@ def shutdown(self, wait=True, *, cancel_futures=False):
             self._process.join()
             self._future_queue.join()
         self._process = None
+        self._future_queue = None
 
     def __len__(self):
         return self._future_queue.qsize()