Merge #11513: [trivial] [tests] A few Python3 tidy ups

Merge #11513: [trivial] [tests] A few Python3 tidy ups

f89308532 [tests] Don't subclass from object for Python 3 (John Newbery)
8f9e3627e [tests] authproxy.py: tidy up __init__() (John Newbery)
323d8f61e [tests] fix flake8 warnings in authproxy.py (John Newbery)
fc0176d01 [tests] use python3 for authproxy.py (John Newbery)

Pull request description:

  A few trivial tidyups in the test_framework:

  - the test_framework can only be run in Python3, so remove the py2/3 compatibility workarounds in authproxy.py
  - while there, do some general tidying up of the module - fix flake8 warnings, make initialization code more compact
  - All classes in Python3 are new-style. No need to explicitly inherit from `object`.

Tree-SHA512: d15c93aa4b47c1ad7d05baa7a564053cf0294932e178c95ef335380113f42e1af314978d07d3b107292a8e3496fd840535b5571a9164182feaa062a1e9ff8b73