stdlib: Fixing erroneous typing in Simulator __init__

The key in the `on_exit_event` dictionary can only be an ExitEvent. The
typing information here suggested a string could be passed. This is not
the case. This patch fixes the issue.

Change-Id: Iaa4b66417c15e31f6aafe9e550a7552461a15be1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/64773
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
diff --git a/src/python/gem5/simulate/simulator.py b/src/python/gem5/simulate/simulator.py
index b32e54a..1d0d3ec 100644
--- a/src/python/gem5/simulate/simulator.py
+++ b/src/python/gem5/simulate/simulator.py
@@ -87,7 +87,7 @@
         board: AbstractBoard,
         full_system: Optional[bool] = None,
         on_exit_event: Optional[
-            Dict[Union[str, ExitEvent], Generator[Optional[bool], None, None]]
+            Dict[ExitEvent, Generator[Optional[bool], None, None]]
         ] = None,
         expected_execution_order: Optional[List[ExitEvent]] = None,
         checkpoint_path: Optional[Path] = None,