Bump testfixtures from 4.7.0 to 6.18.5
Bump testfixtures from 4.7.0 to 6.18.5

Bumps [testfixtures](https://github.com/Simplistix/testfixtures) from 4.7.0 to 6.18.5.
- [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...6.18.5)

---
updated-dependencies:
- dependency-name: testfixtures
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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