⬆️ Bump flake8 from 3.9.2 to 4.0.0 (#120)

Bumps [flake8](https://github.com/pycqa/flake8) from 3.9.2 to 4.0.0.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](PyCQA/flake8@3.9.2...4.0.0)

---
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>

dev-requirements.txt

@@ -5,6 +5,6 @@ chardet==4.0.*
 Flask>=2.0,<3.0; python_version >= '3.6'
 requests>=2.26,<3.0; python_version >= '3.6'
 black==21.9b0; python_version >= '3.6'
-flake8==3.9.2; python_version >= '3.6'
+flake8==4.0.0; python_version >= '3.6'
 mypy==0.910; python_version >= '3.6'
 isort; python_version >= '3.6'