5.2.0rc1

:release-date: 2021-09-07 7:00 P.M UTC+6:00
:release-by: Asif Saif Uddin

• Remove backward compatible code not used anymore (#1344).
• Add support for setting redis username (#1351).
• Add support for Python 3.9.
• Use hostname from URI when server_host is None.
• Use Python's built-in json module by default, instead of simplejson.
• SQS Channel.predefined_queues should be {} if not defined.
• Add global key prefix for keys set by Redis transporter (#1349).
• fix: raise BrokenPipeError (#1231).
• fix: add missing commands to prefix.
• Make BrokerState Transport specific.
• Tests & Docs cleanup.