fix lint error in reachability analysis tool
fix lint error in reachability analysis tool

Summary: This runs in python3 and uses `nonlocal`, which is a python3 feature. Adding this to fix lint error.

Reviewed By: thezhangwei

Differential Revision: D14718889

fbshipit-source-id: a98bc0c1d8a112b2be7d51e24f7445ec4b9d8d46

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