Update flake8 requirement from ^4.0.1 to ^5.0.4

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@4.0.1...5.0.4)

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

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

pyproject.toml

@@ -35,7 +35,7 @@ anyio = "^3.4.0"
 [tool.poetry.dev-dependencies]
 pytest = "^6.2.4"
 mypy = "^0.930"
-flake8 = "^4.0.1"
+flake8 = "^5.0.4"
 black = {version = "^21.5-beta.1", python = "^3.7"}
 mkdocs = "^1.2.1"
 mkdocs-material = "^8.1.4"