Bump flake8 from 3.6.0 to 3.7.1 (#321)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.6.0 to 3.7.1.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.6.0...3.7.1)

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

requirements/flake8.txt

@@ -1,2 +1,2 @@
-flake8==3.6.0
+flake8==3.7.1
 flake8-isort==2.6.0