Issue #13854: Properly handle non-integer, non-string arg to SystemExit

Previously multiprocessing only expected int or str.  It also wrongly
used an exit code of 1 when the argument was a string instead of zero.
diff --git a/Misc/NEWS b/Misc/NEWS
index b832384..6d75b07 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -67,6 +67,9 @@
 Library
 -------
 
+- Issue #13854: Make multiprocessing properly handle non-integer
+  non-string argument to SystemExit.
+
 - Issue #12157: Make pool.map() empty iterables correctly.  Initial
   patch by mouad.