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.txt

@@ -27,7 +27,7 @@ python-dateutil==2.8.1
 repoze.lru==0.7
 requests==2.21.0
 simplejson==3.16.0
-six==1.14.0
+six==1.15.0
 transaction==3.0.0
 translationstring==1.3
 ujson==2.0.3