tests: Remove compatibility code not needed now when we're on Python 3
Summary:
Remove compatibility code not needed now when we're on Python 3.
Backport of core PR12993
https://github.com/bitcoin/bitcoin/pull/12993/files
Depends on D3556
Test Plan:
./test/functional/test_runner.py ../share/rpcauth/rpcauth.py foo
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3557