Replicated, unencrypted metadata is written down differently on py2
vs py3, and has been since we started supporting py3. Fortunately,
we can inspect the raw xattr bytes to determine whether the pickle
was written using py2 or py3, so we can properly read legacy py2 meta
under py3 rather than hitting a unicode error.
Closes-Bug: #2012531
Change-Id: I5876e3b88f0bb1224299b57541788f590f64ddd4