[electrum] progress converting keystore.py to using bytes

Summary:
D14592 and D14593 were minimal changes just to fix a bug. This continues the job of consistently using and returning bytes everywhere in keystore.py.
After this, the remaining str attributes are self.xpub and self.xprv in BIP32KeyStore. Those are base58 encoded ("xpub...", "xprv..."), so converting to bytes is less trivial.

Depends on D14593

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14594