Bump flake8 from 3.9.2 to 4.0.1 (#631)

Bumps [flake8](https://github.com/pycqa/flake8) from 3.9.2 to 4.0.1.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](PyCQA/flake8@3.9.2...4.0.1)

---
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>

Author: dependabot[bot]

requirements-dev.txt

@@ -1,5 +1,5 @@
 attrs==21.2.0
-flake8==3.9.2
+flake8==4.0.1
 flake8-bandit==2.1.2
 flake8-bugbear==21.9.2
 flake8-import-order==0.18.1