Bump flake8 from 3.9.2 to 4.0.1

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

Author: dependabot[bot]

poetry.lock

@@ -13,7 +13,7 @@ python = "^3.9"
 [tool.poetry.dev-dependencies]
 black = "^21.9b0"
 codespell = "^2.1.0"
-flake8 = "^3.9.2"
+flake8 = "^4.0.1"
 invoke = "^1.6.0"
 pep8-naming = "^0.12.1"
 pytest = "^6.2.5"