Bump flake8 from 3.9.2 to 4.0.1

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]>

Author: dependabot[bot]

requirements_dev.txt

@@ -1,7 +1,7 @@
 pip==21.2.4
 bump2version==1.0.1
 wheel==0.37.0
-flake8==3.9.2
+flake8==4.0.1
 tox==3.24.4
 coverage==6.0
 Sphinx==4.2.0