Bump flake8 from 3.9.2 to 4.0.1

Bumps [flake8](https:/pycqa/flake8) from 3.9.2 to 4.0.1.
- [Release notes](https:/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]>

requirements_dev_optional.txt

@@ -14,7 +14,7 @@ pymongo==3.12.0
 # optional test requirements
 tox==3.24.4
 coverage
-flake8==3.9.2
+flake8==4.0.1
 pytest-cov==3.0.0
 pytest-doctestplus==0.11.0
 pytest-timeout==1.4.2