Update flake8 requirement from ~=5.0.4 to ~=6.0.0

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@5.0.4...6.0.0)

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

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

requirements-dev.txt

@@ -1,6 +1,6 @@
 black[typed-ast]==23.3.0
 bumpversion~=0.6.0
-flake8~=5.0.4
+flake8~=6.0.0
 isort~=5.12.0
 mypy==1.3.0
 pytest-cov~=4.0.0