Bump flake8 from 3.9.2 to 4.0.0 (#245)

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:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

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

Author: dependabot[bot]

requirements.txt

@@ -1,7 +1,7 @@
 -e .
 black==21.9b0; implementation_name=="cpython"
 docutils==0.17.1
-flake8==3.9.2
+flake8==4.0.0
 isort==5.9.3
 mypy==0.910; implementation_name=="cpython"
 pre-commit==2.15