Bump flake8 from 3.9.2 to 4.0.0

Bumps [flake8](https://github.com/pycqa/flake8) from 3.9.2 to 4.0.0.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](https://github.com/pycqa/flake8/compare/3.9.2...4.0.0)

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

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

setup.py

@@ -43,7 +43,7 @@ setup(
         'dev': [
             'nose==1.3.7',
             'coveralls==3.2.0',
-            'flake8==3.9.2'
+            'flake8==4.0.0'
         ]
     },
     zip_safe=False