Python Version queue.py

[jpvoigt]

In frescobaldi_app/job/queue.py the line
from enum import Enum , auto references a python 3.6 thing

It should either be corrected in the requirements running frescobaldi 3 (Python 3.5 crashes)
or changed to not use it.

[uliska]

I've already noticed this in #1108. I will remove the use of `auto` which doesn't warrant the requirement of Python 3.6 -- Diese Nachricht wurde von meinem Android-Gerät mit K-9 Mail gesendet.

[uliska]

@jpvoigt could you please test if the fix works (by nature of "works for me" I can't verify it)?