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](benjaminp/six@1.12.0...1.13.0)

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

requirements.txt

@@ -37,7 +37,7 @@ redis==3.3.10
 # requests==2.20.0
 # scikit-learn==0.20.0
 # scipy==1.1.0
-six==1.12.0
+six==1.13.0
 slacker==0.13.0
 twython==3.7.0
 untangle==1.1.1