Bump flake8 from 5.0.4 to 6.0.0 (#313)

Bumps [flake8](https://github.com/pycqa/flake8) from 5.0.4 to 6.0.0.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](PyCQA/flake8@5.0.4...6.0.0)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/requirements_dev.txt

@@ -4,4 +4,4 @@ tox==3.27.0
 tox-travis==0.12
 codecov==2.1.12
 coverage==6.5.0
-flake8==5.0.4
+flake8==6.0.0