[electrum] prefer int.from_bytes over int('0x'+hex, 16)

Summary:
backport of https://github.com/spesmilo/electrum/commit/d7c5949365cf061020e5dfb82ca26171c0692eb6

Depends on D16279

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16300