message: change default Python block serialization to witness
diff: @@ -605,7 +605,7 @@ class CBlock(CBlockHeader):
  super(CBlock, self).deserialize(f)
  self.vtx = deser_vector(f, CTransaction)
  - def serialize(self, with_witness=False):
  + def serialize(self, with_witness=True):
  r = b""
  r += super(CBlock, self).serialize()
  if with_witness:
  