Add support for pickling Limiter

[JWCook]

In v4, the Limiter class has some unpicklable attributes (lock and _thread_local). This is relevant for multiprocessing, which pickles objects before passing them to worker processes. This worked in past versions, but currently raises errors like:

TypeError: cannot pickle '_thread.RLock' object

This could be done by implementing Limiter.__getstate__ and __setstate__ to remove and recreate these attributes (example: https://github.com/JWCook/requests-ratelimiter/pull/129/changes).