serialise singleton construction in FileLockMeta

[dxbjavid]

FileLockMeta.call reads and writes its singleton cache without a lock, so two threads constructing the same is_singleton=True lock for a fresh path both miss the cache and each build their own instance. That quietly breaks the singleton guarantee callers lean on for reentrant locking across instances, so a later reentrant acquire on what should be the same lock can hit the deadlock check in acquire(), and the concurrent writes to the shared WeakValueDictionary are a race in their own right. ReadWriteLock and SoftReadWriteLock already take their _instances_lock around the get-or-create, so this brings the base metaclass in line.