chore: improve filelock maintenance path

[lphuc2250gma]

Summary:

• Add unit tests covering the Timeout exception's reduce/pickling behavior and repr formatting edge cases (e.g., empty lock_file path, non-ASCII paths) to lock in current behavior of the Timeout class in _error.py
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.

[gaborbernat]

LGTM