Fix coverage for fall-through case

Author: avylove

astroid/_backport_stdlib_names.py

@@ -358,4 +358,4 @@
 elif sys.version_info[:2] == (3, 9):
     stdlib_module_names = PY_3_9
 else:
-    stdlib_module_names = sys.stdlib_module_names
+    stdlib_module_names = sys.stdlib_module_names # pragma: no cover