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[bot] <[email protected]>

Author: dependabot[bot]

requirements.txt

@@ -9,5 +9,5 @@ certifi==2019.6.16        # via requests
 chardet==3.0.4            # via requests
 idna==2.8                 # via requests
 requests==2.24.0
-six==1.12.0
+six==1.15.0
 urllib3==1.25.9           # via requests