py3: Fix pickle field.

Merged Andrey Vertiprahov requested to merge noc-py3-pickle-fields into master

Fix pickle field:

In [3]: from noc.sa.models.managedobjectprofile import ManagedObjectProfile
In [5]: mop = ManagedObjectProfile.get_by_id(84)
In [6]: list(mop.metrics)
Out[6]:
[b'\x80',
 b'\x02',
 b']',
 b'q',
 b'\x01',
 b'(',
 b'}',
 b'q',
 b'\x02',
 b'(',
 b'X',
 b'\x0b',
 b'\x00',
 b'\x00',
 b'\x00',

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