Instances of binascii's methods hexlify, unhexlify, and a2b_hex have been replaced with the build-in bytes module's hex and fromhex methods where appropriate to make bytes <-> hex-string conversions consistent across the functional test files and test_framework.

Also update some code in a comment in serialize_tests.cpp that was using binascii. This code also needed additional changes to work with python3 (because in python 3 you cannot join bytes using a str separator).

This is a backport of core#22619

ninja check-functional-extended