Allow application/x-msgpack deserialization again

There is stacktrace all over the place about refusing to msgpack deserialization [1] for some reason. Our default configuration is supposed to already allow it.

The doc says it supports both the name and the mimetype [2]. So as a tryout/short in the dark, here we go.

It's not tested though.

• [1]

ContentDisallowed('Refusing to deserialize untrusted content of type
msgpack (application/x-msgpack)')

• [2] https://docs.celeryproject.org/en/stable/userguide/configuration.html#std-setting-accept_content

Related to #3632 (closed)

Test Plan

tox still happy

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Migrated from D6410 (view on Phabricator)