[tests] remove isort from tests

isort tests looks to heavy and demands a wrong sorting order
https://integration.wikimedia.org/ci/job/pywikibot-core-tox-docker/15376/console
(probably uppercase first)

Do not include isort to flake8 tests but add it to dev-requirements.txt
to be runned manually.

See also T132122

Bug: T166950
Change-Id: I8f0b16f07151eb0fc1efab341d6c773f8ed57351