Update flake8 requirement from ~=4.0 to ~=6.0

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@4.0.0...6.0.0)

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

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

requirements-test.txt

@@ -1,6 +1,6 @@
 -r requirements.txt
 asynctest~=0.13
-flake8~=4.0
+flake8~=6.0
 flake8-docstrings~=1.6
 mypy==0.961
 pylint~=2.14