[electrum] move pubkey_from_signature -> ECPubkey.from_signature65

Summary:
Depends on D16354

This is split out of D16301

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16355