Bump testfixtures from 6.18.0 to 6.18.1
Bump testfixtures from 6.18.0 to 6.18.1

Bumps [testfixtures](https://github.com/Simplistix/testfixtures) from 6.18.0 to 6.18.1.
- [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/6.18.0...6.18.1)

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

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