* modify zadd calls for redis-py 3.0
redis-py 3.0 changes the zadd interface that accepts a single
mapping argument that is expected to be a dict.
https://github.com/andymccurdy/redis-py#mset-msetnx-and-zadd
* change FailedQueue.push_job_id to always push a str
redis-py 3.0 does not attempt to cast values to str and is left
to the user.
* remove Redis connection patching
Since in redis-py 3.0, Redis == StrictRedis class, we no longer
need to patch _zadd and other methods.
Ref: https://github.com/rq/rq/pull/1016#issuecomment-441010847
* Replaced async keyword with is_async in the Queue class to fix reserved keyword syntax errors in Python 3.7
* Updated tests to use is_async keyword when instantiating Queue objects
* Updated docs to reference is_async keyword for Queue objects
* Updated tox.ini, setup.py and .travis.yml with references to Python 3.7