Update flake8 requirement from ~=3.9 to ~=4.0

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@3.9.0...4.0.1)

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

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

requirements-test.txt

@@ -1,8 +1,8 @@
 -r requirements.txt
 asynctest~=0.13
-flake8~=3.9
+flake8~=4.0
 flake8-docstrings~=1.6
-mypy==0.910
+mypy==0.930
 pylint~=2.12
 pylint-strict-informational==0.1
 pytest~=6.2