gh-126688: Reinit import lock after fork

[colesbury]

The PyMutex implementation supports unlocking after fork because we clear the list of watiers in parking_lot.c. This doesn't work as well for _PyRecursiveMutex because on some systems, such as SerenityOS, the thread id is not preserved across fork().

• Issue: CPython should not assume that pthread_self returns the same value in fork parent and child #126688