Build(deps): Bump six from 1.14.0 to 1.15.0

Bumps [six](https://github.com/benjaminp/six) from 1.14.0 to 1.15.0.
- [Release notes](https://github.com/benjaminp/six/releases)
- [Changelog](https://github.com/benjaminp/six/blob/master/CHANGES)
- [Commits](benjaminp/six@1.14.0...1.15.0)

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

requirements-test.txt

@@ -7,4 +7,4 @@ coverage==4.5.4
 pytest-cov==2.8.1
 flake8==3.8.3
 freezegun==0.3.12
-six==1.14.0
+six==1.15.0