Previously TestZBlk was xfailing as e.g.

        --- FAIL: TestZBlk/py3_pickle3 (0.00s)
    panic: ZBlk0(0000000000000002): loadBlkData: wendelin.bigfile.file_zodb.ZBlk0(0000000000000002): activate: pysetstate: expect str; got ogórek.Bytes [recovered]
            panic: ZBlk0(0000000000000002): loadBlkData: wendelin.bigfile.file_zodb.ZBlk0(0000000000000002): activate: pysetstate: expect str; got ogórek.Bytes

because on py3 ZBlk data is saved as bytes while zblk.go code was trying
to decode it as bytestring (str from py2).

-> Fix that by adjusting ZBlk/go to follow py3 model with ZBlk data being
   considered to be bytes but also accepting bytestr from py2 for that.

Now on go side loading ZBlk works for all py2_pickle{1,2,3} and
py3_pickle3 data.