Jenkins just runs tox which has the env/commands to run defined in tox.ini at the root of the repo. The relevant section for the linter is:

[testenv:pep8]
deps = flake8==2.5.5
commands = flake8

It is run on Jessie which has python defaulting to python2.7 hence flake8 is run under python2.7 and use its AST parser. Thus when one does:

#!/usr/bin/python3
import abc

class RemoteExecution(metaclass=abc.ABCMeta):
    pass

That indeed fails because metaclass=abc.ABCMeta is invalid python2.7 syntax.

flake8 could probably be made smarter by detecting the inspected file has a python3 shebang and have it shell out to something like python3 -m flake8 whatever.py. But I don't think it support that.

A lame workaround is to setup a linter environment for each python version. And exclude/include files as needed. It is not so nice and going to be annoying long term, but on a short term perspective that will unlock that patch.

Change 327746 had a related patch set uploaded (by Hashar):
Support flake8 with python3

https://gerrit.wikimedia.org/r/327746

Change 327746 abandoned by Hashar:
Support flake8 with python3

Reason:
On https://gerrit.wikimedia.org/r/#/c/326155 , Volans recomends to move the file transferring system to a standalone repository. Would indeed be easier to finely tune the test and maybe even package it :D

https://gerrit.wikimedia.org/r/327746

Three years later, is this still a problem? If yes, is move the code out of puppet.git to a standalone repo the preferred solution and should that be a subtask which blocks this task? If yes, would someone create that? If no, how to proceed? Asking as I'd like to not have tickets with "stalled" status forever. Thanks! :)