[3.14] gh-140263: Fix data race in test_lock_two_threads (gh-140264) (gh-140369)

Clang-20 detects a data race between the unlock and the non-atomic
read of the lock state. Use a relaxed load for the assertion to avoid
the race.
(cherry picked from commit f11ec6e643f54f4ee698f7dfc878812a315f2af4)

Co-authored-by: Sam Gross <colesbury@gmail.com>