Bump six from 1.12.0 to 1.13.0

Bumps [six](https://github.com/benjaminp/six) from 1.12.0 to 1.13.0.
- [Release notes](https://github.com/benjaminp/six/releases)
- [Changelog](https://github.com/benjaminp/six/blob/master/CHANGES)
- [Commits](https://github.com/benjaminp/six/compare/1.12.0...1.13.0)

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

requirements.txt

@@ -22,5 +22,5 @@ python-dateutil==2.6.1
 requests==2.22.0
 s3transfer==0.1.13
 setuptools==40.3.0
-six==1.12.0
+six==1.13.0
 transip==2.0.0