Bump flake8 from 3.5.0 to 3.7.9

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.5.0 to 3.7.9.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.5.0...3.7.9)

Signed-off-by: dependabot-preview[bot] <[email protected]>

requirements-dev.txt

@@ -1,6 +1,6 @@
 coverage==4.4.2
 ddt==1.2.0
-flake8==3.5.0
+flake8==3.7.9
 mock==3.0.5
 pytest==3.6.3
 pytest-cov==2.8.1