Bump flake8 from 3.9.2 to 4.0.1 (#423)

Bumps [flake8](https://github.com/pycqa/flake8) from 3.9.2 to 4.0.1.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](https://github.com/pycqa/flake8/compare/3.9.2...4.0.1

)

---
updated-dependencies:
- dependency-name: flake8
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.dev.txt

 # astroid and pylint raising false positives:
 # https://github.com/PyCQA/pylint/issues/3139
 pylint==2.10.2
-flake8==3.9.2
+flake8==4.0.1
 mypy==0.910
 nose==1.3.7
 pytest==6.2.5