Bump flake8 from 3.5.0 to 3.8.3

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.5.0 to 3.8.3.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.5.0...3.8.3)

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

requirements.txt

@@ -10,7 +10,7 @@ djrill==2.1.0
 ipython==6.2.1
 boto==2.48.0
 moto==0.4.31
-flake8==3.5.0
+flake8==3.8.3
 djangorestframework==3.11.0
 djangorestframework-httpsignature==1.0.0
 djangocms-admin-style==1.2.7