test: [refactor] Fix F841 flake8

contrib/devtools/clang-format-diff.py

@@ -164,7 +164,7 @@ def main():
                 'Failed to run "%s" - %s"' % (" ".join(command), e.strerror)
             )
 
-        stdout, stderr = p.communicate()
+        stdout, _stderr = p.communicate()
         if p.returncode != 0:
             sys.exit(p.returncode)