Flake8 reports wrong severity level for code Fxxx

[lcheylus]

Flake8 uses Fxxx for error codes generated by PyFlakes, see https://flake8.pycqa.org/en/latest/user/error-codes.html and https://flake8.pycqa.org/en/latest/glossary.html#term-error-code

Currently, these error codes are reported with the Warning severity level by Flake8 plugin instead of Error level, same issue as #220 for E999 code.

For now, I have to keep pycodestyle, PyFlakes and McCabe plugins in my LSP client configuration instead of using Flake8 plugin, to have the correct Warning/Error codes.

[jhossbach]

You're right, it should be sufficient to include all F codes as errors and only consider E999 as error. I'll make a PR for that

[lcheylus]

I'm currently pushing my PR for this issue.