Bump flake8 from 3.6.0 to 3.7.1

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.6.0 to 3.7.1.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.6.0...3.7.1)

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

setup.py

@@ -4,7 +4,7 @@
 EXTRAS_REQUIRE = {
     "tests": ["pytest", "mock", "pytest-mock", "scripttest==1.3"],
     "lint": [
-        "flake8==3.6.0",
+        "flake8==3.7.1",
         "flake8-bugbear==18.8.0",
         "mypy==0.660",
         "pre-commit==1.14.2",