Update flake8 requirement from ^3.9.2 to ^4.0.0

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...4.0.0)

---
updated-dependencies:
- dependency-name: flake8
  dependency-type: direct:development
...

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

pyproject.toml

@@ -26,7 +26,7 @@ toolforge = {version="^5.0.0", optional=true}
 
 [tool.poetry.dev-dependencies]
 mypy = "^0.910"
-flake8 = "^3.9.2"
+flake8 = "^4.0.0"
 black = "^21.9b0"
 pytest = "^6.2.5"
 coveralls = "^3.2.0"