Consider changing async to is_async

With release of python 3.7, now async and await are reserved keywords. (What’s New In Python 3.7 — Python 3.11.1 documentation). As python 3+ brings a lot of promises, we should be changing async to is_async in background jobs.

Why “is_async”: This is from RQ worker, which changed its “async” parameter to “is_async” in its queue initialisation.