Update to flake8 4.0.1.

[lukaszgo1]

Link to issue number:

Discussion in #15299
Unblocks PR #15546

Summary of the issue:

The version of Flake8 we're using is extremely old. While it worked well up to now, it does not support some newer syntactic construct in recent versions of Python (in particular walrus operator). Unfortunately we cannot update to the latest release, since it does not support flake8-tabs, nor linting a diff (the latter can be achieved using external tools).

Description of user facing changes

None

Description of development approach

Flake8 is updated to the version 4.0.1. This is the last version which supports flake8-tabs, and it also recognizes various newer language constructs in Python, by the simple fact that it was released almost 2.5 years after the version we're using currently.

Testing strategy:

Ensured that code from PR #15546 can be linted correctly.

Known issues with pull request:

While this version of Flake8 is way more recent than what we we're using up to now, it predates Python 3.11. It means that some constructs (case statements) would not be recognized. Possible solutions are discussed in #15299.

Code Review Checklist:

• Documentation:
  • Change log entry
  • User Documentation
  • Developer / Technical Documentation
  • Context sensitive help for GUI changes
• Testing:
  • Unit tests
  • System (end to end) tests
  • Manual testing
• UX of all users considered:
  • Speech
  • Braille
  • Low Vision
  • Different web browsers
  • Localization in other languages / culture than English
• API is compatible with existing add-ons.
• Security precautions taken.

[seanbudd]

Thanks @lukaszgo1