Pylint: minor code simplifications

Simplify the code in minor ways. Each of this changes fixes a warning
from Pylint 2.4 that doesn't appear with Pylint 1.7.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/scripts/test_psa_constant_names.py b/tests/scripts/test_psa_constant_names.py
index c02555e..2c9f058 100755
--- a/tests/scripts/test_psa_constant_names.py
+++ b/tests/scripts/test_psa_constant_names.py
@@ -453,7 +453,7 @@
     tests.run_all(inputs)
     tests.report(sys.stdout)
     if tests.errors:
-        exit(1)
+        sys.exit(1)
 
 if __name__ == '__main__':
     main()