[Python] Fix bug where WorkerHandlerManager.close_all re-initialized state with incorrect dict type

[hjtran]

WorkerHandlerManager stores cached handlers as self._cached_handlers = defaultdict(str). When you reset the manager with manager.close_all() it resets the cached to just a regular dict instead of a default dict, i.e. self._cached_handlers = {}.

If you try to create another worker handler after calling close_all(), you'll get a KeyError.

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[jrmccluskey]

LGTM, thanks!