bpo-40094: Fix which.py script exit code (GH-19286)

It now uses os.waitstatus_to_exitcode() to convert os.system() exit
status into an exit code.
diff --git a/Tools/scripts/which.py b/Tools/scripts/which.py
index df54ce0..b42e07c 100755
--- a/Tools/scripts/which.py
+++ b/Tools/scripts/which.py
@@ -49,6 +49,7 @@
                     msg(filename + ': not executable')
             if longlist:
                 sts = os.system('ls ' + longlist + ' ' + filename)
+                sts = os.waitstatus_to_exitcode(sts)
                 if sts: msg('"ls -l" exit status: ' + repr(sts))
         if not ident:
             msg(prog + ': not found')