Bump flake8 from 4.0.1 to 6.0.0

Bumps [flake8](https://github.com/pycqa/flake8) from 4.0.1 to 6.0.0.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](PyCQA/flake8@4.0.1...6.0.0)

---
updated-dependencies:
- dependency-name: flake8
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

requirements-dev.txt

@@ -3,7 +3,7 @@
 # tools
 black==23.3.0
 check-manifest==0.49
-flake8==4.0.1
+flake8==6.0.0
 pytest==7.2.2
 Sphinx==4.3.2
 tox==3.25.0