Update flake8 requirement from ~=5.0 to ~=6.1

Updates the requirements on [flake8](https://github.com/pycqa/flake8) to permit the latest version.
- [Commits](PyCQA/flake8@5.0.0...6.1.0)

---
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~=5.0
+flake8~=6.1
 flake8-docstrings~=1.7
 mypy==1.0.0
 pylint~=2.15