[electrum] convert Transaction.update_signatures to take bytes

Summary:
In the next diff, this method will operate on TxInput objects.

Depends on D14504

Test Plan:
python test_runner.py
sign a transaction with a trezor

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14503