build(deps-dev): update flake8 requirement from ^3.9.2 to ^4.0.1 (#155)

Updates the requirements on [flake8](https://github.com/pycqa/flake8) to permit the latest version.
- [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
...

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

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

pyproject.toml

@@ -22,7 +22,7 @@ importlib_metadata = {version = ">=1.6", python = "<3.8"}
 
 [tool.poetry.dev-dependencies]
 black = "^21.9b0"
-flake8 = "^3.9.2"
+flake8 = "^4.0.1"
 flake8-black = "^0.2.1"
 flake8-bugbear = "^21.4.3"
 flake8-isort = "^4.0.0"