Bump flake8 from 3.8.4 to 3.9.0 (#24)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.4 to 3.9.0.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.4...3.9.0)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

setup.py

@@ -43,7 +43,7 @@ setup(
         'dev': [
             'nose==1.3.7',
             'coveralls==3.0.0',
-            'flake8==3.8.4'
+            'flake8==3.9.0'
         ]
     },
     zip_safe=False