flake8: fix various errors

- update config & ignore .tox folder
- remove unused import
7 jobs for !70 with topic/default/project_toml in 17 minutes and 1 second (queued for 5 seconds)
latest merge request
Name Stage Failure
failed
cubicweb-tests Tests
cubicweb/server/test/unittest_checkintegrity.py .....                    [  0%]
cubicweb/server/test/unittest_datafeed.py ...... [ 1%]
cubicweb/server/test/unittest_edition.py . [ 1%]
cubicweb/server/test/unittest_hook.py ........ [ 3%]
cubicweb/server/test/unittest_ldapsource.py .ERROR: InvocationError for command '/builds/cubicweb/RQL/cubicweb/.tox/py3-server/bin/python -m pytest --durations=20 --cov-report xml:/builds/cubicweb/RQL/cubicweb/reports/coverage-server.xml --cov=cubicweb -rs --html=server.html --self-contained-html --junitxml=server.xml --profile --profile-svg /builds/cubicweb/RQL/cubicweb/cubicweb/server/test' (exited with code -9)
___________________________________ summary ____________________________________
ERROR: py3-server: commands failed
Cleaning up file based variables
ERROR: Job failed: command terminated with exit code 1