Bump flake8 from 3.7.7 to 3.7.8 (#372)

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>

requirements/flake8.txt

@@ -1,2 +1,2 @@
-flake8==3.7.7
+flake8==3.7.8
 flake8-isort==2.7.0