Bump flake8 from 3.7.7 to 3.7.8

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.txt

@@ -122,7 +122,7 @@ invoke==1.2.0
 coverage==4.5.3
 parse-type==0.4.1
 poster~=0.8.1
-flake8==3.7.7
+flake8==3.7.8
 pytest==4.6.3
 pytest-bdd==3.1.0
 splinter==0.11.0