fixup! csfilter-kfp: install a simple man page

Simplify the call to `isatty()`.

src/csfilter-kfp

@@ -198,7 +198,7 @@ def main():
         help="file to store all excluded findings to")
 
     parser.add_argument(
-        "--json-output", action="store_true", default=(not os.isatty(sys.stdout.fileno())),
+        "--json-output", action="store_true", default=(not sys.stdout.isatty()),
         help="produce JSON output (default if stdout is not connected to a terminal)")
 
     parser.add_argument(