[3.13] gh-117657: Acquire a critical section around `SemLock.__{enter,exit}__` (GH-118812) (#118856)

These methods are purely wrappers around `Semlock.{acquire,release}`,
which expect a critical section to be held.
(cherry picked from commit c30d8e5d6c6b657817d6b342f1021676d04dd5af)

Co-authored-by: mpage <mpage@meta.com>