Bump flake8 from 2.6.2 to 3.7.9

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 2.6.2 to 3.7.9.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/2.6.2...3.7.9)

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

Author: dependabot-preview[bot]

requirements_dev.txt

@@ -2,7 +2,7 @@ pip==18.0.0
 bumpversion==0.5.3
 wheel==0.29.0
 watchdog==0.8.3
-flake8==2.6.2
+flake8==3.7.9
 tox==3.14.1
 coverage==4.1
 Sphinx>=1.4,<1.5