Become compatible with flake8 5.0.0

[AlexWaygood]

Fixes #257

[AlexWaygood]

I manually verified that this warning is emitted if the plugin is run in an environment with flake8==4.x installed. Other than that, the test suite still passes for me locally with flake8==4.x installed.

[AlexWaygood]

Thanks both for the reviews!

[sobolevn]

I understand that I am late to the party, but please reconsider this.

flake8 has enough problems alredy:

• It does not play well with importlib-metadata and does not want to do anything about it for 2 years
• It is almost impossible to use correct version resolution for flake8 / importlib-metadata / other packages that require importlib-metadata
• Most likely it will mean that we will have to drop python3.7 support as well

Here's what we do for now:

1. https://github.com/wemake-services/wemake-python-styleguide/blob/60023c9ca95e56197a8af318d7fb88f7a84d9ad5/pyproject.toml#L50-L53
2. https://github.com/wemake-services/wemake-python-styleguide/blob/60023c9ca95e56197a8af318d7fb88f7a84d9ad5/pyproject.toml#L58
3. But it is still not enough: https://github.com/wemake-services/wemake-python-styleguide/actions/runs/3180761218/jobs/5184717583#step:6:34

[AlexWaygood]

Hey @sobolevn! Sorry to hear this has been causing pain :(

Very happy to be flexible here. I understand there's a bit of an importlib_metadata/flake8 incompatibility problem at the moment, and I don't think any of us want to aggravate that.

Could you be specific about what exactly is causing problems from flake8-pyi's end here, and what you'd like to see changed? Then we can see what we can do to fix it :)

[sobolevn]

Requiring flake8>=5.0 will cause importlib-metadata conflicts for all projects using python3.7 and other deps that require recent importlib-metadata versions. Like sphinx, pytest-randomly, and many others that pin importlib-metadata for >=4.3

Definition: https://github.com/PyCQA/flake8/blob/b220ee957aca37ae5919509f6f25dd571a836d52/setup.cfg#L44

[AlexWaygood]

Cool. So would it "fix" it for you if we extended the period before we drop support for flake8<5? Or would you prefer it if we dropped the FutureWarning altogether?

I'd prefer the first option ideally, but either could be doable :)

[sobolevn]

I think that dropping older versions of flake8 is fine when 3.7 is dropped.

[AlexWaygood]

@sobolevn, I've filed #293, to delay removing support for running with flake8 <5 until Python 3.7 is EOL :)