Bump flake8 from 3.6.0 to 3.7.6

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.6.0 to 3.7.6.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.6.0...3.7.6)

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

requirements-test.txt

@@ -2,7 +2,7 @@
 
 coverage==3.7.1
 coveralls==0.4.2
-flake8==3.6.0
+flake8==3.7.6
 mock==1.0.1
 tox==1.7.2
 requests_mock==1.5.2