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](https://github.com/benjaminp/six/compare/1.14.0...1.15.0)

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

requirements.txt

@@ -22,5 +22,5 @@ python-dateutil==2.8.1
 requests==2.23.0
 s3transfer==0.3.3
 setuptools==44.1.1
-six==1.14.0
+six==1.15.0
 transip==2.1.2