Bump flake8 from 3.5.0 to 3.6.0

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.5.0 to 3.6.0.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.5.0...3.6.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot-support

travis/requirements.txt

@@ -5,7 +5,7 @@ pytest-pep8==1.0.6
 pytest-leaks==0.2.2
 pytest-travis-fold==1.3.0
 pytest-pylint==0.13.0
-flake8==3.5.0
+flake8==3.6.0
 python-coveralls==2.9.1
 pylint==1.8.4
 -r ../requirements.txt