psbt: follow-ups: BCDataStream.read_bytes() should return bytes
psbt: follow-ups: BCDataStream.read_bytes() should return bytes

This fixes keepkey, as in particular the code in the plugin expected
TxOutpoint.txid to be bytes not a bytearray (and the TxOutpoint named tuple
itself claims txid to be bytes).

pooler/electrum-ltc,vialectrum/vialectrum,spesmilo/electrum,wakiyamap/electrum-mona,wakiyamap/electrum-mona,pooler/electrum-ltc,spesmilo/electrum,pooler/electrum-ltc,pooler/electrum-ltc,vialectrum/vialectrum,spesmilo/electrum,wakiyamap/electrum-mona,wakiyamap/electrum-mona,spesmilo/electrum,vialectrum/vialectrum