Bump flake8 from 3.7.8 to 3.7.9

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.7.8 to 3.7.9.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.8...3.7.9)

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

Author: dependabot-preview[bot]

requirements-dev.txt

@@ -6,7 +6,7 @@ aiomysql==0.0.20
 aiopg==1.0.0
 coverage==4.5.4
 docker==4.1.0
-flake8==3.7.8
+flake8==3.7.9
 ipdb==0.12.2
 motor==2.0.0
 pytest-cov==2.8.1