* 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