Update flake8 requirement from <4.0.0,>=3.8.3 to >=3.8.3,<7.0.0

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@3.8.3...6.0.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

pyproject.toml

@@ -64,7 +64,7 @@ test = [
     "pytest >=6.2.4,<8.0.0",
     "pytest-cov >=2.12.0,<5.0.0",
     "mypy ==0.971",
-    "flake8 >=3.8.3,<4.0.0",
+    "flake8 >=3.8.3,<7.0.0",
     "black ==22.12.0",
     "isort >=5.0.6,<6.0.0",
     "pytest-asyncio",