Close processes in Work Queue and Task Vine shutdown (#3576)

This releases 2 file descriptors with work queue (from 21 to 19 at the end
of CI Work Queue test) and 4 file descriptors with Task Vine (from 19 to 15
at the end of CI Task Vine test)

This is part of work being merged from draft PR #3397 to shut down components
more cleanly, rather than relying on process exit.

parsl/executors/taskvine/executor.py

@@ -589,11 +589,13 @@ def shutdown(self, *args, **kwargs):
         # Join all processes before exiting
         logger.debug("Joining on submit process")
         self._submit_process.join()
+        self._submit_process.close()
         logger.debug("Joining on collector thread")
         self._collector_thread.join()
         if self.worker_launch_method == 'factory':
             logger.debug("Joining on factory process")
             self._factory_process.join()
+            self._factory_process.close()
 
         # Shutdown multiprocessing queues
         self._ready_task_queue.close()

parsl/executors/workqueue/executor.py

@@ -704,6 +704,8 @@ def shutdown(self, *args, **kwargs):
 
         logger.debug("Joining on submit process")
         self.submit_process.join()
+        self.submit_process.close()
+
         logger.debug("Joining on collector thread")
         self.collector_thread.join()