Bump six from 1.12.0 to 1.15.0

Bumps [six](https://github.com/benjaminp/six) from 1.12.0 to 1.15.0.
- [Release notes](https://github.com/benjaminp/six/releases)
- [Changelog](https://github.com/benjaminp/six/blob/master/CHANGES)
- [Commits](benjaminp/six@1.12.0...1.15.0)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -4,4 +4,4 @@ fusepy==3.0.1
 pycparser==2.20
 pygit2==0.28.2
 raven==6.10.0
-six==1.12.0
+six==1.15.0