Update testfixtures requirement from ^6.18.5 to ^7.0.0

Updates the requirements on [testfixtures](https://github.com/Simplistix/testfixtures) to permit the latest version.
- [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/6.18.5...7.0.0)

---
updated-dependencies:
- dependency-name: testfixtures
  dependency-type: direct:development
...

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