Hash calculated incorrectly for transactions

[garethjns]

In transaction class, hash is calculated from incomplete header that contains only the first input and output. This needs to be updated to handle all inputs and outputs.

from pybt.py3.block.Transaction:

    def prep_header(self) -> bytes:
        """Return transaction header bytes.
        Only works for single input and output transactions for now
        """
        header = self._version \
            + self._nInputs \
            + self.txIn[0]._prevOutput \
            + self.txIn[0]._prevIndex \
            + self.txIn[0]._scriptLength \
            + self.txIn[0]._scriptSig \
            + self.txIn[0]._sequence \
            + self._nOutputs \
            + self.txOut[0]._value \
            + self.txOut[0]._pkScriptLen \
            + self.txOut[0]._pkScript \
            + self._lockTime

        return header