Avoid using pytest-flake8 plugin

[juhoinkinen]

Closes #599.

Unpins flake8. On my laptop version 5.0.4 get installed, while 6.0.0 was released two days ago.

In the CI/CD pipeline I added flake8 step that both installs flake8 from PyPI with pip and runs it. There version 6.0.0 get installed. I did not find a good GH Action for flake8 (an official one or any which seemed otherwise reputable), so it can be better to install it this way.

The mismatch of the versions is unfortunate, but makes no difference at this time.

Also I'm thinking that it might be a good idea to switch installing also isort and Black from PyPI (instead of using the GH Actions), and using Poetry for that (regular dependencies can be omitted by poetry install --only dev).

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

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[codecov[bot]]

Codecov Report

Base: 99.54% // Head: 99.55% // Increases project coverage by +0.00% :tada:

Coverage data is based on head (2e812ec) compared to base (1a58116). Patch has no changes to coverable lines.

Additional details and impacted files

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[osma]

Looks very good!

Also I'm thinking that it might be a good idea to switch installing also isort and Black from PyPI (instead of using the GH Actions), and using Poetry for that (regular dependencies can be omitted by poetry install --only dev).

I think that sounds like a good idea! Would it make sense to do this change already in this PR, or a new one after this has been merged?

[juhoinkinen]

Looks very good!

Also I'm thinking that it might be a good idea to switch installing also isort and Black from PyPI (instead of using the GH Actions), and using Poetry for that (regular dependencies can be omitted by poetry install --only dev).

I think that sounds like a good idea! Would it make sense to do this change already in this PR, or a new one after this has been merged?

Better to make a new PR for that think, it might need some experimenting to make it work...