Bump testfixtures from 4.7.0 to 6.15.0
Bump testfixtures from 4.7.0 to 6.15.0

Bumps [testfixtures](https://github.com/Simplistix/testfixtures) from 4.7.0 to 6.15.0.
- [Release notes](https://github.com/Simplistix/testfixtures/releases)
- [Changelog](https://github.com/Simplistix/testfixtures/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/Simplistix/testfixtures/compare/4.7.0...6.15.0)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
modoboa/modoboa-radicale,modoboa/modoboa-radicale,modoboa/modoboa-radicale