gh-108520: Fix bad fork detection in nested multiprocessing use case (#108568)

gh-107275 introduced a regression where a SemLock would fail being passed along nested child processes, as the `is_fork_ctx` attribute would be left missing after the first deserialization.

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Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>