Bump six from 1.14.0 to 1.15.0 (#202)

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>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.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==5.4.2; python_version>="3.5"
-six==1.14.0
+six==1.15.0
 tornado==5.1.1; python_version<"3.6"
 tornado==6.0.4; python_version>="3.6"
 trio==0.13.0; python_version>="3.6"