Bump flake8 from 3.7.7 to 3.7.8

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.7.7 to 3.7.8.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.7...3.7.8)

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==0.16.0
 coverage==4.5.3
 docker==4.0.2
-flake8==3.7.7
+flake8==3.7.8
 ipdb==0.12
 motor==2.0.0
 pytest-cov==2.7.1