Bump flake8 from 3.7.6 to 3.7.7 (#332)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.7.6 to 3.7.7.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.6...3.7.7)

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

requirements/flake8.txt

@@ -1,2 +1,2 @@
-flake8==3.7.6
+flake8==3.7.7
 flake8-isort==2.6.0