Avoid using pytest-flake8, run flake8 separately instead

[osma]

We are currently using pytest-flake8, which runs flake8 code style checks within pytest. But it has turned out to be fragile (see PR #598 and tholo/pytest-flake8#87) and many projects are switching to a setup where flake8 is run separately (see pypa/pyproject-hooks#147 , pytest-dev/pytest#9217 (comment), miketheman/pytest-socket#82 )

We should also consider doing this. At the moment it is not possible to use the recently released flake8 version 5 because it's not compatible with pytest-flake8 1.1.1, so we have for now pinned flake8 to version 4 in #598, but this is not a good long term solution.

[juhoinkinen]

Now (since #640 and #641) we have a CI/CD job for Black and isort lint checks, so the flake8 check could be easily added to that job.

The instruction for the precommit hook already includes flake8, so running flake8 within pytest is redundant, and pytest-flake8 plugin can be just removed and flake8 upgraded to latest version.