Close stdin used to send config to Interchange

This was introduced in PR #3463 and at the time I incorrectly assumed that
interchange exit would close both ends of the pipe. That is untrue.

For example: pytest parsl/tests/test_htex/ --config local
ends with 341 fds open before this PR,
and 327 file descriptors open after this PR.

parsl/executors/high_throughput/executor.py

@@ -551,6 +551,7 @@ def _start_local_interchange_process(self) -> None:
         logger.debug("Popened interchange process. Writing config object")
         stdin.write(config_pickle)
         stdin.flush()
+        stdin.close()
         logger.debug("Sent config object. Requesting worker ports")
         try:
             (self.worker_task_port, self.worker_result_port) = self.command_client.run("WORKER_PORTS", timeout_s=120)