[test framework] simplify bytes to hex encoding

Summary:
Since Python 3.5, byte objects have a method for hex encoding.

Inspired by core#20916

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12153