Arq

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* prevent duplicate ``job_id`` when job result exists, fix 137
* add "don't retry mode" via ``worker.retry_jobs = False``, fix 139
* add ``worker.max_burst_jobs``

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* improved error when a job is aborted (eg. function not found)

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* fix semaphore on worker with many expired jobs

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* add support for different queues, 127 thanks tsutsarin

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* use dicts for pickling not tuples, better handling of pickling errors, 123

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* use ``pipeline`` in ``enqueue_job``
* catch any error when pickling job result
* add support for python 3.6