metadata: accept only fixed python-xattr in python3

metadata: accept only fixed python-xattr in python3

This is currently broken on python3, it returns junk when
we pass bytes because it uses string and %s internally.
I made a fix for it, so we can detect here if it's fixed
(in which case the NS_USER constant is bytes, not string),
load the module only if it is indeed fixed.

We can do this test always since in python2 bytes == str
and thus isinstance('user', bytes) == True.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>