Correction 1

John White, has discovered a flaw in the descriptions above.
This flaw is caused by the extensions attribute:

tx = {'ref_block_num': 36029,
      'ref_block_prefix': 1164960351,
      'expiration': '2016-08-08T12:24:17',
      'operations': [['vote',
                      {'author': 'xeroc',
                       'permlink': 'piston',
                       'voter': 'xeroc',
                       'weight': 10000}]],
      'extensions': [],
      'signatures': [],
      }

In the OP, we did not serialize this extensions such that our
serialized sequence was too short. Since we do not make use of
extensions here, and the serialization is a length-prefixed array,
we can fix the flaw by simply appending 0x00 to the serialization

    buf += (varint(len([]))

Everything else should stay the same.