Bump six from 1.12.0 to 1.13.0 (#152)

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>

Author: dependabot-preview[bot]

requirements.txt

@@ -6,7 +6,7 @@ pytest-tornasync==0.6.0.post2; python_version >= '3.5'
 pytest-trio==0.5.2; python_version>="3.6"
 pytest==4.6.6; python_version<"3.5"
 pytest==5.2.2; python_version>="3.5"
-six==1.12.0
+six==1.13.0
 tornado==5.1.1; python_version<"3.6"
 tornado==6.0.3; python_version>="3.6"
 trio==0.12.1; python_version>="3.6"