Update mixer requirement from ==6.0.* to ==6.1.*

Updates the requirements on [mixer](https://github.com/klen/mixer) to permit the latest version.
- [Release notes](https://github.com/klen/mixer/releases)
- [Changelog](https://github.com/klen/mixer/blob/develop/Changelog)
- [Commits](klen/mixer@6.0.0...6.1.3)

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

requirements/test.txt

@@ -3,7 +3,7 @@ pytest-cov==2.5.*
 pytest-django==3.2.*
 pytest-pythonpath==0.7.*
 factory-boy==2.12.*
-mixer==6.0.*
+mixer==6.1.*
 isort==4.3.*
 flake8==3.5.*
 pep8==1.7.*