msgpack is throwing a warning that the unpacker has deprecated the use
of encoding argument. Instead the unpacker uses `raw=False`, which
does an unpack with similar behavior to the old encoding='utf-8'
mechanism.
This will reduce a lot of warning spam for projects using msgpack
such as Keystone. The change to the kwarg "raw" from "bytes_as_raw"
happned between 0.5.1 and 0.5.2. Changing the lower constraint is
required to ensure the new arguments are matching the method
signature.
Change-Id: Iadbee3ec8def9512369a415fb4603dc05d0cbc56
d5855294a8