lib/zodb: tests: Mark test_zstor_2zurl as xfailing on py3

Because it uses NEO/py which is not yet ported to py3 and fails even with today's NEO/py master (v1.12-148-g1020ac40) with:

=================================== FAILURES ===================================
_______________________________ test_zstor_2zurl _______________________________

...

>       _(neo("test",  "127.0.0.1:1234"),         "neo://test@127.0.0.1:1234")            # NEO/ip4

lib/tests/test_zodb.py:526:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
lib/tests/test_zodb.py:497: in neo
    from neo.client.Storage import Storage as NEOStorage
../../neo/src/lab.nexedi.com/kirr/neo/neo/client/__init__.py:52: in <module>
    from . import app # set up signal handlers early enough to do it in the main thread
../../neo/src/lab.nexedi.com/kirr/neo/neo/client/app.py:31: in <module>
    from neo.lib import logging
../../neo/src/lab.nexedi.com/kirr/neo/neo/lib/__init__.py:17: in <module>
    from .logger import logging
../../neo/src/lab.nexedi.com/kirr/neo/neo/lib/logger.py:25: in <module>
    from .util import nextafter
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    from __future__ import division
    import os, socket
    from binascii import a2b_hex, b2a_hex
    from datetime import timedelta, datetime
    from hashlib import sha1
>   from Queue import deque
E   ModuleNotFoundError: No module named 'Queue'

../../neo/src/lab.nexedi.com/kirr/neo/neo/lib/util.py:22: ModuleNotFoundError

After this patch running wendelin.core test on py3 locally passes fully with FileStorage, and fully with ZEO if https://github.com/Pylons/zodburi/pull/35 fix is applied to zodburi.

/cc @jerome, @levin.zimmermann, @vnmabus