Bump flake8 from 3.7.4 to 3.7.5

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.7.4 to 3.7.5.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.4...3.7.5)

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

test-requirements.txt

@@ -16,7 +16,7 @@ coverage==4.5.2           # via pytest-cov
 cryptography==2.5         # via pyopenssl, trustme
 decorator==4.3.2          # via ipython, traitlets
 entrypoints==0.3          # via flake8
-flake8==3.7.4
+flake8==3.7.5
 idna==2.8
 immutables==0.6           # via contextvars
 ipython-genutils==0.2.0   # via traitlets