Bump six from 1.12.0 to 1.15.0 in /cvat/requirements (#1742)

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](https://github.com/benjaminp/six/compare/1.12.0...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>

cvat/requirements/testing.txt

@@ -2,5 +2,5 @@
 fakeredis==1.4.1
 # Fix dependencies for fakeredis 1.1.0
 # Pip will not reinstall six package if it is installed already
-six==1.12.0
+six==1.15.0
 coveralls