Remove unused key in iteration of task dictionary (#2863)

This comes from a flake8-bugbear warning.

parsl/dataflow/dflow.py

@@ -1146,8 +1146,11 @@ def wait_for_current_tasks(self) -> None:
 
         logger.info("Waiting for all remaining tasks to complete")
 
-        items = list(self.tasks.items())
-        for task_id, task_record in items:
+        # .values is made into a list immediately to reduce (although not
+        # eliminate) a race condition where self.tasks can be modified
+        # elsewhere by a completing task being removed from the dictionary.
+        task_records = list(self.tasks.values())
+        for task_record in task_records:
             # .exception() is a less exception throwing way of
             # waiting for completion than .result()
             fut = task_record['app_fu']