PYTHON-4347 Ensure client can be opened after fork() (#1682)

Author: ShaneHarvey

pymongo/mongo_client.py

@@ -904,6 +904,7 @@ def target() -> bool:
         # this closure. When the client is freed, stop the executor soon.
         self_ref: Any = weakref.ref(self, executor.close)
         self._kill_cursors_executor = executor
+        self._opened = False
 
     def _after_fork(self) -> None:
         """Resets topology in a child after successfully forking."""